There's no reason why you can't build a 4 legged "duplexer", but not with just a pair of bpbr duplexers. You'd need dual circulators and band pass cavities for each transmitter, a set of notch filters for each transmitter frequency in front of each receiver, a set of notch filters for each receiver frequency on each transmitter, and other cavities as dictated by specific tx and rx performance levels, etc.
Another approach is a pair of window filters (expensive) tee'd together, with each repeater using it's own bpbr duplexer. Dual circulators would still be required.
I won't forget to mention that you would need a scalar network analyzer for measurements, know how to use it, and be handy with a Smith chart to calculate the critical length cables that would be needed to tie it all together.
One thing you won't likely get here is the length's of the critical length cables soe you can just hook stuff together and call it good.
Barring all that complication, as mentioned by others, two antennas would be easy. If there's a separate level or location for receive antennas, use a multicoupler for sharing a receive antenna, and make a transmitter combiner for the two transmitters.
The concept of combining lots of transmitters and receivers onto a single antenna isn't at all unusual. Trunking systems can have as many as 28 channels TX and RX combined, multicoupled, and duplexed onto a single antenna, although there is usually several TX antennas. I've also done 700, 800, and 900 onto a single broadband antenna. Works good.
