They should be the same chemistry (like flooded or AGM or GEL). Preferably they should be the same brand (because even flooded chemistries will differ a little between brands). They should be the same age (because charge/discharge characteristics will change with age). Of those the first one is the only one I would really worry about.
Switching banks used to be the normal, however since the life of the battery bank is non-linear with depth of discharge (deeper discharge = shorter life) the consensus now is to parallel them into one large bank for longest life.
I would consider putting fuses at each end of the connecting cable. Either end can source fire starting currents if the other end gets loose....
