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I think they mean why can you lift say 50kg with both arms, but can only lift 20kg in each individually, rather than 25kg individually.

Why can both hands at the same time lift more than what you can when you add together what you can lift with your left and right hand

Yep

Why can both hands at the same time lift more than what you can when you add together what you can lift with your left and right hand

"Left hand separately + Right hand separately is less than Left and right hand at the same time" is what he means

Because two hands better utilizes back and leg muscles, and you have better balance.

2nd sentence because this sub is dumb and one sentence for eli5 is apparently not sufficient.

You’re able to use multiple muscle groups together instead of isolating a single group. Instead of just relying on the arm, you now have chest and core muscles helping to stabilize and balance.

Stability. When lifting things with your individual hands/arms there is a lot of expended energy spent preventing horizontal movement. When the thing you're lifting is connected across your body there is much less wasted energy.

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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.

no

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This is a sentence too long, can I get a tldr?

If someone with one hand can at most lift 20 lb, with two hands they can probably lift at least 50 lb, which is more than the sum of the combined hands alone.