The Shure UA221 Antenna Splitter/Combiner Kit includes two adapters that can be used as one–input, two–output or two-input, one-output passive antenna splitter/combiners for UC4, SLX4, ULXS4, ULXP4, U4S and U4D receivers.
eBay
UA221 splitters are intended for use with Shure Wireless Systems. Model: UA221. (2) Single-input, dual-output passive antenna splitters. (2) BNC male-to-male adapters.
eBay
The Shure URT2 is the correct rack tray for mounting two Shure PGX4 wireless receivers. In fact, the Shure URT rack tray is also compatible with PG88 and PG4 receivers as well as P6T and PA765 antenna combiners.
eBay