Ok, this may sound kinda stupid, but what will happen, if you instead of 12V and 90mA will add to device (cooling fan) 6V and 180mA?
Just in case you mean, what happens if you replace a 12V fan with a 6V fan, that's not a good idea, because the 6V fan will draw too much current from a 12V power supply. It will run faster, generate more noise and probably burn out quite quickly (if not immediately!)
You can do the reverse, put a 12V fan in place of a 6V fan, but it will run at lower power and not deliver so much air.
I have happily done this in a non-critical situation, replacing a 6V fan with a larger 12V fan, which ran more slowly and therefore much more quietly, but seemed to generate almost as much airflow and the cooling was adequate for my purpose.
(I have often since thought that using larger slower fans would be a good idea. So many computers seem to use tiny fans running at high speeds, generating lots of noise and wearing out their bearings in a couple of years. But they are presumably constrained by space.)
If you are replacing something like a CPU fan, you'd better stick to the same Voltage and at least the same size.