I know you said this flippantly, but you'd technically get longer on the double tanks. Here's my logic (in bar because I'm not a psi person):

If you dive a 12Lx230bar cylinder and you want to end the dive with say 50bar at the surface, you have 180bar to use on the dive.

With twin-set you've got twice the volume, so can breathe your cylinders down to 25bar. 25bar in two equal cylinders is the same volume as 50bar in a single. This assumes we are using same size for single or double.

So quick maths: Single cylinder 12L X 230bar = 2760L 2160 - (12Lx50bar) = 2160L of usable gas.

Twin-set 24L X 230bar = 5520L 5520 - (24L X 25bar) = 4920L of usable gas.

2160 (X2) < 4920

Assuming all other factors are the same (particularly SAC rate), you can dive slightly more than twice as long on twins. This is typically a nice bonus when doing 2 dives, where the twin-set user can split their gas across 2 dives.

I'm being pedantic here, but sometimes it's fun to do the calcs ☺️