Slater-
If your existing batteries are fairly new, adding two more is pretty much OK, the differences in them won't be great. If they are old, or don't come up to full voltage, then by all means the new ones should be in a separate bank.
But if you can survive with using the existing batteries until they are shot, and then you replace the entire bank with one newer larger bank, you will have a better system overall and get a better return on your
money.
Just something to think about, not knowing your circumstances or the battery condition.