Either use CuBitcrack or ClBitcrack for;
1x RTX™ A6000 (total 48 GB GPU Memory, 10,752 CUDA cores)
2x RTX™ 3090 (total 48 GB GPU Memory, total 20,992 CUDA cores)
1) The same amount of GPU memory, which GPU can get more MKey/s?
2) Between GPU Memory or CUDA core, which is an important factor for speed in MKey/s?
As you could use 2x RTX 3090 you will search twice as fast as in the A6000.
Memory is not a very important metric, I believe Bitcrack has a memory ceiling of about 4GB of GPU memory or so.