Same basic deal as the phone/device pairing - the headsets have a pair/discover mode that you put them in at the same & they link right up. The Scala G4 supports 3 bikes simultaneously (or 2 driver/pass pairs, I think) and the Sena supports 4 total w/ manual channel switching. How exactly you pair multiples I couldn't tell ya, but I think it's just a matter of doing the pairing cycle for each combination of units.
As far as range, the old Scala I have does about 1/8~1/4mi line-of-sight & a couple hundred yards in the curves (with an intermittent distortion which gets quite annoying). The G4 advertises 'up to one mile', but I've read the effective range is more like 1/2mi or so, and they have similar complaints as the old ones - not enough volume or signal quality.
http://www.cardosystems.com/scala-ri...er-g4-powerset
The Sena advertises 900 meter (.56mi) range, but I've read that it's a SOLID km+ range with up to a mile line-of-sight, with much better signal/audio quality than any other unit. The construction/control seems superior (Sena is primarily an industrial device mfg & it appears more rugged/ergonomic than others). Though it doesn't have the built-in FM radio, it has A2DP bluetooth (higher-fidelity stereo) so paired w/ a decent MP3/radio with bluetooth (like the Samsung P3) it has MUCH better music capability - loudest/clearest on the market, supposedly.
http://www.senabluetooth.com/
Yeah, I've been researching/considering this setup a bit lately...
