Do you need to control 2 units to activate Back to Back, or can you activate it with only 1 unit under the 'do as much as you can' rule?

Ruling: You must control 2 units to activate Back to Back. The card requires exactly two targets ("give two friendly units"), and you cannot activate it with only 1 unit.

Nuances:

• Cards that instruct players to do something (like Cull the Weak or Acceptable Losses) don't target and follow "do as much as you can" on resolution
• Back to Back requires choosing/affecting specific units, which makes them targets
• If a card says "up to" you could choose zero, one, or two targets, but without "up to" you must meet the exact requirement
• Activating Back to Back with only 1 unit would be a general error requiring a rewind
• A unit cannot back itself up (must be two different units)