If i control fiora peerless and opponent enter my battlefield with 2 unit and i suddenly starcross one of their unit will it become double might

No — Fiora won't double her Might.

Fiora's ability is a triggered ability (Rule 383.4.f.2.b): the "one on one" requirement is checked only at the moment she gains the Defender designation, and only once per combat. With two attackers there at that instant, the condition failed, so the trigger never went on the chain — removing one attacker afterward can't retroactively create it (FAQ #11502).

One extra wrinkle with your specific card: Star-Crossed returns a friendly unit and an enemy unit to hand — so if Fiora's your only unit there, she bounces too, and she isn't even on the battlefield to be doubled. Either way, the answer is no.