Without knowing what specific type of lift you are talking about, here are just three ways off the top of my head with something like a bicep curl.

1. A bicep curl with a dumbbell can do a pretty good job at isolating the lift to only use the bicep, especially if you are seated with your elbow on your knee. If you are doing a standing two handed bicep curl with a barbell, you have the ability to engage other muscles, particularly at the start to overcome inertia. You can engage your shoulders and back to help get the bar moving. This overcoming inertia is the hardest part of the lift as it requires extra force just to get it moving.

2. Along these same lines, if one arm is slightly stronger than the other, then when using two hands this arm could begin moving the barbell slightly which helps the other arm in overcoming inertia to start moving.

3. this is more specific to a slight increase in two handed… let’s say you can do bicep curls with a 30lb dumbbell, but you can’t go up to 35lb. But if there was a 31 or 32lv dumbbell, maybe you could. So with both hands, you could possibly do 65 lbs, because your arms working together adds in that slight little bit more each could do.