Does Back to Back require two valid targets to be played, or can it be cast with only one friendly unit?

Ruling: Back to Back requires two valid targets to be played. You cannot cast it if you only control one friendly unit.

Nuances:

• The card text says "give two friendly units" without "up to" language, making both targets mandatory
• Cards like Flash that say "up to 2" do not require the maximum number of targets
• If one target becomes invalid before resolution, the spell still resolves on the remaining valid target (do as much as you can applies at resolution)
• First Mate works differently because the readying effect is triggered by an ability after First Mate is already played, not a targeting requirement for playing the card itself