I have two units attacking a bf. On my first sand soldier i have hand hammer equiped. On my second one i have eye of the herald. Can i use hand hammer first in order to get +2 then use eye of the herald effect ?

Timing error

No — you can't bank the +2. Hand Hammer's bonus is a static ability, not something you "use," and Eye of the Herald's trigger will kill the condition. Here's what happens: Eye of the Herald's "When I move" trigger is mandatory and fires as soon as its unit moves. It resolves before the showdown opens. The Recruit token lands at the moved to battlefield (FAQ #11204/#11375). When both Sand Soldiers move to the battlefield, the Hand Hammer unit does not sees exactly one other friendly unit. The other Sand Soldier and one Recruit Unit are present aswell. Once that 1-Might Recruit exists, Hand Hammer's condition — "exactly one other unit you control" — is no longer true (now it's two other units), so the +2 turns off. Static bonuses are continuously re-evaluated, so a removal of one of the other units could activate it.