Untap up to two target creatures. They each get +2/+2 until end of turn.
If Synchronized Strike targets two creatures and one becomes an illegal target before it resolves, the remaining target creature will still be untapped and get +2/+2.
Synchronized Strike can target an untapped creature. It will still get +2/+2.
You can cast Synchronized Strike with two, one, or zero targets.
You can’t target the same creature twice to have it get +4/+4.