With a combi boiler, things will only get worse - maybe you should consider changing the heating system so you can have stored hot water.
Not a 2 in one - I think your builder was suggesting 2 separate showers, one of each.
How many people in your house? If its just a couple of you then a boiler shower will be OK - you'll know when the other person is in the shower and not to use the water. If you have a family though, with the shower being used a lot in the morning / evening then a boiler shower is likely to...
Having gone through 2 electric showers, and having similar issues, I fitted a power shower similar to these
http://www.screwfix.com/c/bathrooms-kitchens/showers/cat820272#category=cat820332&sort_by=price
The additional price has been more than covered in the lower electricity bill (Daughters...