Hi JDW, kindly noticed that only phone devices are able to connect to the same phone at one time. Multiple PC's is not available at present.
Also, only one user can access to the SMS messages on the PC instead of phone devices. An account can only be in a computer landing.
Tips: you can visit this FAQ for more information: [Help & FAQ] Multiple Device Support http://forums.airdroid.com/viewtopic.php?f=4&t=15912
Hope it helps.