To add to what Peter has already said, you may also find that if you are using the electric shower and someone starts to use your existing mains fed shower that the drop in pressure will knock out the heater on the electric shower for a time. This is to prevent the heating element from over heating the water (which is now flowing slower through the element) and burning you.

If your water pressure is good you may be ok, but if you think about it you will be taking quite a lot of water with both showers running at the same time.

Your plumber is right about having 2 mixer mains fed showers in the same system, especially off a combo boiler. I have 2 in our house (fed from a tank actually) and if you use both showers at the same time only the one on the middle floor gets the hot water, the ensuite goes cold as the pressure is released through the path of least resistance. I was told this would happen when it was plumbed in, the solution is not to use the 2 showers at the same time.

Cheers

Cris