can i temporal breach on a battlefield a when temporal breach is hidden on battlefield b ? can i choose a unit on the battlefield a ?

No. A Hidden Temporal Breach can only choose its target from the battlefield where it was hidden (CR 811.1.d.2) — so one hidden at battlefield B can only banish a unit at battle B, never a unit at battlefield A.

Temporal Breach just says "Banish a unit" with no location restriction, so the general Hidden targeting rule applies and there's no exception (unlike Tideturner, whose own targeting restriction is impossible at its battlefield). If there's no legal unit at B, you can't play it from Hidden at all — but you can still cast Temporal Breach normally from hand to banish a unit at A.