I'm likewise a 6'3" newb (35" inseam/PBH) trying to sort out the whole fit issue.

I think it'd be great to be able to borrow a bike, or maybe even a few, and take them out for a longish test ride but of the handful of people I know with bikes, none of them are all that close to being my size. So I feel like I'm going to be kind of stuck making an educated guess.

The two articles I've found most helpful so far have been the competitivecyclist.com's Fit Calculator and this handy essay on Choosing a Frame Size (mentioned above). And browse around sheldonbrown.com (start here).

The Competitive Cyclist's calculator offers me three different fit styles (and explains them in detail) that range from about 58 cm to about 61 cm. Rivendell's PBH chart puts me in the neighborhood of 60 to 62 cm for a 700c wheel. CC's fit styles are smaller for racers and larger for a more relaxed riding style. The Rivendell guy seems to think most people are riding on bikes that are too small for them and he makes a case for his ideas about frame size. I think that Sheldon's site mentioned both inseam *.65 and inseam *.67 but I can't find the link at the moment. Those put me in the 58 to 60 range.

If I were you I'd read through those and take some measurements. After looking at those three sources I'm getting an idea with a centerpoint and a bit of space around it. I'm looking at 58 to 62 and figuring that if anything, I probably belong towards the higher end of that range. You might be built very differently than I am or 63 cm might be at the larger end of what you'd want. A bit smaller might be workable or maybe even more appropriate.

If you're firmly attached to the idea of sharing the bike, run both of your measurements through and see if there's any overlap… maybe it'd be possible to meet in the middle in a way that's not too uncomfortable for either of you. Zonatandem's analogy to sharing shoes is probably still worth considering, but then again what sort of riding are you planning on doing? Is this for long distance rides or shorter hops around town?