If opponent has 1 unit on forbidding waste and i attack. On his focus he plays a unit into forbidding waste. Does the defend -2 still applied?

No — once the second unit hits Forbidding Waste, the -2 stops applying immediately.

Forbidding Waste's ability is a static "while" effect that re-evaluates continuously (FAQ #10921). The instant your opponent's unit is officially present, the defender is no longer "alone," so the -2 is gone — even though the showdown is already underway. Just track the unit's current Might correctly at the Combat Damage Step, since that step uses the values present at that time (Rule 460).