This commute is doable, but will be painful (I have friends who are doing this and after 1 year decided to just to have their own places, albiet they are both in surgical fields with crazy hours). The options are: have two places, live in SF (which gives your SO the horrible commute but you get to live in the city which is much more entertaining), live around Stanford (which gives you the horrible commute and you're living in suburbia, although it is much nicer weather), or live in between (still good weather but you'll both suffer commutes and you and your SO has to be okay with it). B/c radiology has great hours, it means you start and end during rush hour (excluding call days/nights). To get to most of the UCSF sites from Stanford area at 8am takes about 1.5 hours (more so for the less commonly rotated sites), so signing up for that would be miserable and a huge time suck.
In my opinion you'd realistically just have to live in between, leave at like 5:45-6am, get to whatever hospital you're at in 30ish minutes and go to the gym/study/sleep. You'll have to brave the traffic in the evening, b/c it doesn't die down until 7-8pm and that'd be a lot of time waiting around. So if you're okay with this, rank it highly. If not, you two will probably have to put that combination lower in your couples match (if you are couples matching).
Source: I did an away at UCSF and lived in Stanford area with family