I have multiple profiles set up on my Garmin 820: Road and Hybrid. I switch to whichever profile applies to the bike I'm riding (I have a Synapse road bike, and a Quick CX hybrid). The GPS keeps mileage totals for each profile. So does the Connect app. So I can review, on the GPS, the mileage for each profile, which for me means for each bike. The 820 is not different from the 520 or 520+ in this respect, I believe.

So absolutely you can keep track of mileage per bike. If you find that you need more than one riding profile per bike, that's fine too. You could create Road-Race, Road-Train, Hybrid-Commute, Hybrid-Offroad, and Hybrid-Casual, for example. In that case, to get the total road bike miles you would have to add up Road-Race and Road-Train (for example).

Another solution that may be acceptable to you is to put a cheap-o cycling computer on each bike. Because I sometimes loan my hybrid to my brother so he can ride along with me while I'm on my road bike, I put an inexpensive CatEye cycling computer on the hybrid bike. That way I can keep track of mileage on that bike even if it's being ridden by someone else, so that I can keep track of when it starts to become time to measure the chain, inspect shift cables, etc. Also that inexpensive computer allows him to see his speed. I don't have a cheap computer on my road bike because nobody aside from me ever rides it, and I always attach the GPS when I'm riding. If you didn't need the cycling computer for its display while riding, you could stick it out of the way on the down tube; it's just there to track overall mileage anyway, right?