1. To use just the combi, look at the hot water flow rate for the combi (litres per minute), divide by 1.7 and that is roughly the flow rate you will get at each shower. Subject to:
1a. You having a mains incoming flow rate of (calculated shower flow rate x 2 x 1.4) to provide hot and cold at the same time.
1b. You don't run any other outlets / flush any toilets while both showers are running.
1c. The calculated flow rate gives you the performance you require at each shower.
2. The other options you have are:
2a. Make one shower an electric one with a cold mains feed only. Won't be the world's greatest shower experience and needs additional cable running, but will give you hot water if the combi is out of action.
2b. Fit an unvented hot water cylinder. Use the combi as one source of heat for this via zone valve(s). You could run one or both showers off this, or run one directly from the combi. Get a cylinder with immersion heater(s) fitted and again you can have hot water even if the combi is broken. Unvented cylinder can be fitted anywhere subject to loading. Needs an incoming pressure of 2.0 bar and a minimum flow rate of around 25 lpm. Relatively expensive and require annual service. Must be installed / registered / serviced by someone with current G3 qualification.
2c. Fit a vented (traditional) hot water cylinder. Will need cold water storage cistern in loft, and be situated below CWSC, e.g. traditional airing cupboard. May well need pumps for one or both showers. Probably cheaper than unvented, and maybe less fuss to install depending on space / location.
3. Before you do anything, measure or have measured:
3a. The standing pressure of your incoming cold water supply. Requires a pressure gauge. Easiest done on an outside tap.
3b. The dynamic pressure of your incoming cold water supply. Same a 3a, but with one mains tap running. Typically done on an outside tap, with the kitchen cold tap running.
3c. The flow rate of your incoming cold water supply. Can be done with a weir cup, or by timing the flow into a 10 litre bucket. Looking for litres per minute. Should be done from a cold mains connected outlet without restrictive valves in place.
3d. The pressure and flow are best measured several times, at different times of the day and different days of the week, as pressure and flow rate vary with neighbourhood usage.
4. If the dynamic pressure is less than 1 bar, or the flow rate less than 15 lpm, you will have difficulty getting the best from the combi.
