There are tons of opinions out there...the most common of which is that if you're going with a certain speaker size/type, stick with that as drivers of varying size end up running out of phase and colliding sound and being overall bad juju. So 2 210's or 2 115's or 2 1288's, etc.
That being said speakers like the 1288 bridge that gap of the 210+115 setup in that they are internally baffled and crossed over, similar to the Fearful cabs (please note I said "similar to"), and feature multiple sized and voiced drivers.
Personally, I like the 1288 with my streamliner 900. 2 1288's sound even better too, and even more dynamic with the 2 12's and the 4 8's. However, the Uber 1288 pairs quite nicely with the Uber 115 as well...with my Streamliner 900 and at volume. The challenge is that the wattage handling is different...it takes more to drive the 1288 than it does the 115...so the volumes aren't identical coming out of the 2 cabs. At low volumes this is really noticeable and so I usually practice with just one cab and when live I turn the horn all the way off on the 1288 and just run the horn on the 115. Gives me the entire spectrum of growling, focused mids and deep wide open lows. If I was doing just rock I would go with 2 115s, just jazz would be 2 1288s, but we do a little of all of that with some folk and country mixed in and I switch between fretted and fretless often enough.
Your ears are going to be the best test no matter what anyone else says. If you like the 210's then adding another will give you more volume, slightly lower extension and still be quite articulate. there's nothing wrong with a well matched 210 and 115, in fact that can sound really good, but watch out for the power ratings and make sure they are as close as possible...meaning uber 210 with uber 115, etc.
Btw, I'm looking for a good deal on a 1288...