I think they call it multiplexing. The interrogator can search, lock and track several transponders simultaneously. This radar pulses are microseconds long, only...
A single DME interrogator can track 2 DMEs for FMGC position (one for onside FMGC and one for offside one) plus a different one for display, plus one coupldmto the ILS and you still have one left.
VOR can't multiplex, so if FMGC is calculating position with VOR DME IRS, you must select it for display in RADNAV or see TUNE ZZZ in the scratch pad.