If your concern is to have a backup system (ie a cylinder of water heated by an immersion heater) available if (when) the combi fails, why not use an instantaneous electric shower fed from the mains cold supply.
If it's because a combi does not deliver sufficient hot water for your needs, then the question is whether a combi is really the right choice for you and your household.
If you wish to take advantage of multiple heat sources, then maybe set up your two supplies with , say, the kitchen fed by the combi, main bathroom from cylinder, second bathroom from combi etc. etc.
As Tony said, the two systems operate at entirely different water pressures, so operating any mixers will be problematic at best, and possibly downright dangerous (Images of the combi backfeeding mains pressure hot water into cylinder, which may burst, or the hot water ends up in the cold storage tank, which overheats and collapses)