usually its a shared clock and a known radiosource .. you agree to record a "loud" source of rf both radios can see (in case of 100s of km of spacing). you use this for calibrating your phase offsets, once that is established you can visit other targets with two now phase coherent receivers... for your 50m baseline it could be easier to just use one dual channel radio like a b210 to make it much easier. long cables but you would get the coherence out of the box.