I have what might seem like the nicest problem in the world: I've got offered funding for both of my dream PhD's. I am stunned, incredulous, gobsmacked, but also deeply confused: both are with supervisors I would absolutely love to work with, both are topics that genuinely fascinate me. Having to choose one feels like Sophie's choice to me - as soon as I imagine choosing one I start grieving the other. My question is, would it be madness to switch to part-time mode and pursue both? One is funded by ESRC and the other one, by AHRC; there are parallels in the subject matter but the projects and research methods are completely different, so there is no way I could unite the two (although my two prospective supervisors know and extensively quote each other). The field is humanities (cultural/visual studies in one case, same but with a good dash of social sciences in the other). Thank you!
Andrew mostly covered why this is administratively impossible, but I think it's also worth addressing the misunderstanding of what a Ph.D. is embedded in this question. Pete Clark said it better than I can, but Ph.D.'s are not merit badges. More is not better. Getting a Ph.D. on one of these topics will not stop you from working on the other. Working with one of these advisors officially will not stop you from staying in touch with the other, and potentially collaborating in the future. You want to have ideas about other stuff to work on once you finish your dissertation.
I don't envy having to make the choice, and you will have to close one the doors part of the way to actually finish your degree. But you don't have to slam it all the way shut, and you never know when ideas you know thought you had set to the side will come right back to you.
From an academic standpoint, I'm pretty sure this is a bad idea - but as I don't have a PhD I'll leave it to those who do to explain why!
From an administrative standpoint, the answer seems to be "no, you can't do this" - or, at least, you can't do it and still get both funded positions.
If you're seriously considering this I'd strongly advise you talk to your institution(s) student offices to be sure - but from a quick skim through the ESRC postgraduate funding guide:
If a student already holds an award from, or is otherwise financially supported by, another organisation and the ESRC considers that award, or other form of support, to be sufficient to cover maintenance and/or tuition fees, the student will not be eligible for an award from the ESRC.
Students who have already received government funding for Master’s-level or PhD-level training may apply for further funding from an accredited DTC but the total length of funding available plus previous government funding will not normally exceed four years for full-time study (or the part-time equivalent).
Students who receive other governmental studentship support are not eligible for an ESRC award.
I would assume that the other six research councils have more or less identical policies. This guide is for students funded through a DTC; again, though, I suspect the basic policies are the same for any other type of studentship.
While it doesn't specifically address your situation, it does seem to say - at least to me - that they won't fund a PhD if you're already receiving funding from elsewhere. You might be able to finesse it if one was unfunded, or had fees-only funding from a non-RCUK source.
Unfortunately, I think you're going to have to make a rather sharp decision in the next few days - good luck, and I don't envy you it.
From the academic standpoint, this might not be completely absurd, but, IMHO, it is rather pointless...
The most important part is that PhDs are usually hard. If you are good enough to properly do two at the same time, you could redirect this talent, and effort, into a stellar phd, which helps you to get a proper job as PI and then research whatever you want, because with a stellar phd, you would be a rockstar anyway :)
Part-time made me smile. It makes little sense to speak about working hours when referring to a PhD. In the end you need one thesis in order to graduate. One, not two halves. If you are going to deliver the same research in a much longer time, with more probabilities that something goes wrong, you can imagine the position of the institution. So what about instead of doing two PhDs in parallel, doing one after the other? ...but wait a second, after the first PhD you can get a much better position (and curriculum) as a postdoc! Wouldn't you go for that?
That is to say that brilliant people get many offers and have to take choices. You cannot fully know all the consequences, so the motivations may appear weak and you may be worried about mistaking. However choosing and going down a path, whatever it is, is much better than getting stuck at the fork, which is the only thing you should regret.
OK, all the other answers have been negative, but why not.
My idea is that a PhD takes 40-50 hours of your time up per week. On busy weeks it can be 60, but generally, 40-50 for three years completion.
If you have enough self-discipline, I am sure you can do it!
Just set out different days you'll work at each University if possible.
Teaching may be tricky, but nothing that you can't work around.
The funding issue they mention is possibly a problem, but not definitely.
Best of luck, I am sure you're not the first person to do it.
From a purely practical point of view, if you are open with your funding agencies and they allow you to keep the funding for both PhDs, then I would keep both options on the table, and evaluate them by exploring what you think is right for you as an individual - you know yourself the best.
There is the argument that you are depriving someone else of PhD funding by holding two scholarships. But equally it could be argued that, because you are spending an extra 3-4 years as a PhD student (time that, under the one-PhD scenario, you would spend as a postdoc), during those last 3 years you are effectively opening up Postdoctoral funding to someone else. So perhaps the ethics balances out.
I'm speaking as someone who got funding to do a year long exchange to another laboratory, followed by a Masters, both of which occurred within my PhD. They have effectively added 2 years and a few months to my PhD project, and this extra time has definitely been well spent. I know that you might be looking at an extra 3 or 4 years if you choose to do both.
There is no rule that says all PhD students have to have the same or similar experience, and doing two PhD's, if you choose to and think it is the right thing for yourself, would definitely make you unique and would help increase the diversity of academic experiences that exist.