I have both a 212 and 2-210 cabs. For a long time I had just the 212, then added the 210 as my small rig, then added a second 210 for a BIG rig. I was doing the 2x210 rig for a little, but went back to my 212 for my current gig.
All IME...
The advantage of the 2x210 rig is that you have two rigs. A small one for small gigs, rehearsals, home practice, jams, etc. And a big rig that would cover pretty much any gig or any situation - and in your case you are already half way there.
A good 212 rig will cover pretty much most everything - mostly. All things being equal, the 2x210 will be a bit louder, and that can be nice for big outdoor shows, really big stages, and gigs with no FOH support. - I would say that a 2x210 will provide more thud than the 212 equivalent -
But assuming there is a PA, the 212 should be fine in those situations. The thing the 212 is not great for is the little coffee house gig, or acoustic jam with friends. It's fine cuz you can turn down, but it's kind of big. A 210 is nice for that, and you never know what situations might pop up.
As far as schlep, lot's of guys preach modular is better. For me, it's kid of a toss up. Sure the 212 is heavier, but it's not that heavy or big - and it's one thing to load instead of two. The weight of the two rigs is actually pretty close to the same, but the 212 is smaller. So when I think about trips from the car, times through a door or down a funny hallway, going up stairs - the 212 might weigh more, but in ways, it's easier and less awkward. Note that my 212 is tall/skinny. If it was a box (like a 410 shape), I don't think I would feel the same. Really, neither one is a bad schlep.
Very honestly, if I were in your shoes, I would get a second matching 210 - unless you are in a situation where you are chasing tone and want a "different flavor." A 2x210 is pretty hard to beat in terms of a do-it-all situation.