The top lever has very little to do with it.

The top lever is there to OPEN a closed action.

The locking mechanism below the top lever is there to lock an action when it is closed.

Actually, those who know how to close a doublegun properly hold the barrels firmly, say stationary, in one hand and with the other hand holding the wrist of the stock, bring the stock up smoothly until the action snaps closed, and know why.