I have a thermo bar shower which requires min 1.5 bar. My cold supply is high pressure mains fed, which is high enough to operate my shower. Hot water supply is gravity fed from loft, which is low pressure. Can a twin imeller pump take a cold water feed from the mains. Or should i just fit a single impeller pump to the hot water feed.
Neither. You will have unbalanced supplies which will make your showering experience less than satisfactory. Also you are not allowed to pump mains.
I assume you are suggesting that you tee off the cold from the rising main which goes to your CWST in the roof (which I am expecting you to have if your HWC is gravity). You should not do that. You will need to empty the CWST and fit a new outlet connection at low level. Use this via your pump soley for feeding the shower.
Your hot feed for the shower needs to be taken off the cylinder in such a way as to a) provide a totally independent feed from other drawoffs and b) prevent the pump sucking air as well as water. Feed this via the other half of the pump.
Now you have balanced supplies at equal pressure to feed your shower.