Combine grp 24 with grp 27 batteries?

Have you performed a power audit to determine what you need? What are your intentions? Will a second group 24 get you there?

I agree with Don. Going with a pair 24s of the same make would be better. If a pair of 24s doesn't get you there then adding a 27 would be marginally better and they won't play well together as they age.