Comet calls these "duplexers", but they are also referred to as combiners or splitters. They are basically band-pass filters, simultaneously passing RF in both directions through the common connector while the band-pass filters provide a high degree of isolation between ports.
* TX and RX from two radios at the same time into a dual-band antenna
* Combine a dual-band radio with separate outputs into a dual-band antenna
* Separate a single connector duplexed radio into separate antenna systems
* Allow a single coax run into two antennas at the top of the tower, and two radios at the bottom by using a duplexer at both ends of the coax run, avoiding cost and weight.
Choose from versions with a variety of band-pass frequency ranges, coax leads or no leads, and connector combinations.