You've got a couple options. You can either get an amp with enough input channels to support everything and with at least one output channel to get the sound back to the main mixer, or you can get a mixer and run the final mixer output into your amp and into the main mixer.
The kind of amp you get depends on how much gear you have and how many outputs you use. Honestly, if you're using more than about two outputs, and with the amount of gear you seem to have, you probably are, I wouldn't even try looking for an amp with a ton of inputs. I'm not sure they even exist. Mine (Roland KC-350) has 4 channels, although one doesn't get played from the amp's output, so I can use it for a click track. Basically, I can connect up to 3 outputs from my gear; with two keyboards, that's all I need. There are other amps that support a few channels; your best bet is to go to a music store and try out a few on the same keyboard.
As for using a submixer (the term for using a secondary mixer for some of the parts that are played through the house speakers), there's a ton of choices. You'll have to try out brands or go to a music store for advice, but you've generally got two types of mixers: some are either rackmount (if they're really small) or are built like an amp (well protected case, the control surface is vertical, not horizontal) and can take up space in a rack or sit on top of an amp, and some are smaller ones designed to be used on a desktop (or, if the top of your amp is large enough and flat, that would work too). What you go for depends on cost and on your setup: you may not want your amp right beside you (many people like it behind them) but you'll almost certainly want the mixer right beside you so you can make small adjustments during the gig. If you have a rack mount or amp-like mixer, you could have the rack in front of you or find something to put your mixer on so you can reach it without it being in the way and still have your amp behind you. With a desktop mixer, you need the flat surface.
