muscles output

In biomechanics, it is necessary to specify the particular muscle(s) you are measuring before making or testing a claim like this. If you are simply lifting objects around the house, when you use both arms, you typically also use the back and possibly the buttocks. Try your test on a Nautilus type machine to see if what you say is true there.

Time plays a role in the work done by any action.

Double the force for half the time would be more or less the same as half the force double the time.

Therefore using 2 legs would be better if you need continues power, and using both together better if you need bursts of power.

If lifting using a single leg, consider whether the lifting leg is also lifting the leg that isn't being used, as a deadweight. In this particular circumstance, lifting with 2 legs will be more than twice as effective as lifting with one. So it depends. Where is this thread going?