I started a PhD program in the US, which is a standard 5-year US grad program, and soon after starting it (recently), I received an offer for a PhD position I'd applied for in Europe (in Germany), which is a standard 3-year thesis-only European PhD.
How should I ask my advisor in Europe to allow me to do a "joint PhD" with my current university in the US? He told me before I get the offer in my early contacts with him that if I decide to start the PhD later (say, for completing a project, coursework, etc. elsewhere) he's ok with that and flexible regarding the start date of my contract (there's a default start date in the current contract though). But to wait until I get to candidacy in the US and then go to work with him, I'd need to ask him to freeze my offer for almost 2 years. That sounds too long of a deferral to me and I don't want to make him change his mind and tell me something like "Stay there then and apply again two years later if you want"! What is a good way of sharing my thoughts with him, explaining what I think the ideal path is for me, and asking him to help me walk that path? I also have the option to go to Europe for a couple of months, start working with him and while I'm there either make arrangements for returning to the US until I get to candidacy or just withdraw from my current program and finish my PhD there. Would it be a better idea to leave this conversation for soon after starting the PhD in Europe or is it better to have it now?
The issue is that there is no on-going collaboration between the two groups and, as far as I know, not even the two departments. Moreover, a joint PhD is not defined in any of the programs, so it'd be an odd procedure. I know that the European institution is very open to outside collaborations, it actually actively encourages that, and it's not at all uncommon for PhD students there to find external collaborators on their own and even get financial and administrative assistance to go and spend some time in the corresponding institution and work with them.
Further explanation about the situation:
Since I prefer the advisor in European, I'd strongly prefer to do my thesis there, especially because I don't have an advisor in the US yet and I may not be able to work with the professor I'd like to work within my US department as he already has reached his usual limit for PhD students and won't have any graduated for a few years My advisor in Europe knows that I've already started a PhD program in the US and I've told him that I'm ready to go there and start the PhD in his group instead.
Now, given the structures of the programs, and the facts that the two departments (my US PhD, and the European one) are actually working in areas that are closely related, there is a lot of shared interests, and their approach could be very productively combined with each other, I'm thinking of the possibility of doing my PhD in both of the institutions through a joint arrangement. That would be the ideal option for my academic (and even personal) well-being. That way, I hope that I'd get to complete the coursework and other aspects of the doctoral training (such as TAship) at my US institution, but do the major part of my thesis in the group of the advisor in Europe, while being co-advised with my preferred advisor at my current (US) institution. In that case, the US professor may be more likely to work with me too as he'd have to spend less time on advising me. Also, the issues regarding the distance with my significant other will be solved significantly more smoothly (this is the personal aspect).
P.S. I actually don't care about an official "joint PhD" with a degree from both institutions. I'm ok with a regular PhD degree from either one of the institutions. My main purpose for doing this is to complement the research I'd be doing in the European group while working on my thesis, with the training I'll receive in the US, and also take advantage of the enrichment the work with the faculty in the US would bring to my research. So, I don't mind just dropping out of the US department after 2-3 years as long as there's some guarantee I'll have the position in Europe, or possibly, doing most of my thesis work in Europe in an informal setting but getting the degree from my current department in the US.
P.P.S. My department in the US has no idea about this yet and I'm not worried about that since I think their role becomes important when I reach candidacy. Am I wrong about that?