Blaze on and figure it out for others to follow. Not many have incorporated their own wireless mics, although recording wireless feeds not their own is relatively common. There will be teething pains.
I'd want two, spaced apart to get stereo room/ambience/audience. The SBD may or may not be mono, but either way, a good wide stereo ambiance is going to sound best.
There will be some delay due to the wireless transmission, but there is likely to be more delay from the time it takes for the sound to travel through the air from PA the recording position. Best to use a single four channel recorder and shift track position or delay afterward to sync with the SBD. Using two 2-ch recorders is less optimal for sync'ing afterward, but easier in regards to cabling before, during and immediately after recording.. but only because the two separate recorders needn't be co-located. However since the intent is to eliminate the wires that reasoning becomes less compelling. Go with a 4 channel recording device.