I dunno about suffocating, but they shouldn't be anywhere near each other, and bagged separately.
When I send corals, I double bag or triple bag everything, each bag rubberbanded then put into the next bag, rubberbanded, etc. Then I have those bags floating around the container in which I pack everything, but I try to secure them in with styrafoam or newspaper or whatever so though they're floating they shouldn't move too much.
As far as suffocation, I'm considering dry/moist shipping everything I send ever again, because apparently if taken care of properly when they arrive at the new location (thrown into tank in high flow), then they're supposed to have better outcomes than corals shipped in water. Sounds like a dangerous idea, but I've read numerous times about improved shipment success when shipped dry/moist than wet (in RK mag [E. Borneman), in "Coral Propagation" book [Calfo], etc)