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 ?

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 before combat damage.

Here's what happens:

• When both Sand Soldiers move to the battlefield, the Hand Hammer unit sees exactly one other friendly unit (the other Sand Soldier), so it does have +2 (4 Might) at that moment.
• Eye of the Herald's "When I move" trigger is mandatory and fires as soon as its unit moves. It resolves during the showdown — before the Showdown closes and combat damage is assigned (Rule 348, 465). The Recruit token lands at the attacked battlefield (FAQ #11204/#11375).
• 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 ordering the triggers doesn't lock in the bonus.

Net result: you get the +2 only until the Recruit resolves, and by the Combat Damage Step Hand Hammer is giving nothing. If you want the +2 to matter for combat, you'd need to keep the Recruit from being there — there's no way to sequence this so you keep both.