With Protoss there is arbiters, And usesing them for offense is a much seen strategy, but to send two into two diferent bases may seem crazy, it gets you the upper hand on the game. Make sure both your arbiter have energy to recall. Send them to 2 diferent opposing bases(if there is 2) and make sure you go in fast to the part where minerals are being made. Recall a force into that base, and wait untill he/she trys to kill your arbiter, once your arbiter is low on life, use the other to recall your force to the second base. This will kill there probes, drones, or scv's that are getting minerals. You can more than likely kill that base your in now. So send any other forces you have to the other base, it will have good defense, but it cant recover from your attack fast, so send in more and more, and the game is yours.

CreepColony's Opinion: Recall takes some time to do, so your low-life arbiter will probably die.