I dont think it would matter if they where close together since neither are magnetic and moving the disc away from the bridge will mess up the sound. You want those discs near the bridge.
Don't know wich kind of rod type piezo your using but most of them require a pre-amp to even generate sound so your going to need one anyway, might as well search for one that allows you to add a passive pickup like the disc piezo. Don't know if realy switching between pickups is possible but there are pre-amps out there that have optional volume controls wich you could use to switch back and forth between your rod piezo and the disc. Maybe add a kill switch to both volume controls to get that switching effect by turning one pickup off.
In that case your good to go Scotty! The 2 piezo's close together should work just fine as far as un-preamped piezo's can go without feedbacking wich they tend to do with high volumes.