I am a physics student in the UK.
A university (university A) has made me a PhD offer. The email from admissions was very clear that there is a 30-day time limit on this offer. As it happens I am likely to accept the offer, but do wish to take advantage of these 30 days. This is because my partner has also applied to university A, and their application is going well, we think it probable that we may both get offers from this university. However, there is a second university (university B) who has already offered my partner a place and will get back to me in 24 days.
In fact, we will both accept the first of A or B to make us both an offer. Perhaps this is not the best reason to chose a PhD place, but that is beyond the scope of my question.
The professor who would be my supervisor at university A has emailed me to encourage me to accept A's offer sooner. Clearly, I wish to remain on good terms with this professor, in case I do end up at university A. I'm not sure that telling A about the two-body problem is a good idea. How can I politely tell A that I will not accept yet?
Edit; I did inform A of my reasons for hesitating. Not only were they very sympathetic, they did their best to get my partner interviewed sooner, and helped them find an intern-ship with the relevant people.