I try to max-out the GPU usage.
As I only have mid-range GPUs, it's always only one task per GPU.
They are then at 95-100%.

You can use two tasks per GPU to get the time between ending and starting a new task not wasted but I think this results only in some single percent in total.

Your GPU can run as many tasks simultaneously as they fit in its VRAM.
It doesn't make a difference in running one or multiple.
If one task alone needs 1min and already uses 100% of the GPU, then by using 5 tasks, one needs 5min.
In the other way, if this one task uses only 20%, then by using 5 tasks, your GPU gets the wanted 100%.

Honestly it's a long time ago I've tried multiple GPU-Tasks but from the logical side this should be the behavior... :)