Most PLOs utilize 0+/- 6 dBm for 10 MHz reference input. I have used a simple quartz oscillator (low-cost versions are available for < $10.- from Digi-Key, etc.), with an attenuator added to obtain ~ 0 dBm (usually the quartz modules generate ~+10-+13 dBm). To switch to external (other than my) reference source, I simply connect the "output wire" to another SMA connector, and use a switch to power my 10 MHz reference, or, switch it off.
Equally easy is to feed two PLOs from one internal/external 10 MHz source. Only some PLOs are radiating signals back to their reference inputs, so I would recommend to use a 10 MHz power splitter with some isolation, like Mini-Circuits ZMSC-2-1, and adjust the output power from 10 MHz source to get ~0 dBm to each PLO reference input.