Hi
You generally need 20l per minute at the kitchen cold tap for good water flow through the house.
But If you’re pressure is low it’ll affect showers, sinks etc. regardless.
Two mixer showers on at the same time is quite a lot for most combis to cope with also. You’ll need a decent sized combi and good mains water pressure to run two mixers at a time.
Best thing to do is see what flow rate you’re getting through the taps and go from there. You’ll need a flow cup or get a plumber to check.

Dear customer
Having two showers on at the same time with flow rate of 10l/min each and temperature at the outlet 40°C (and cold water temperature 12°C) will require heat source with output of 39kw, 7l/min each - 27.4kw. Your boiler, as per manufacturer's instructions, has 28kw heat output. There might be a flow regulator in the boiler (the manufacturer doesn't say) that would limit the flow of water through boiler to about 14l/min (so it falls within the maximum heat output). Check if you have at least 7l/min on two cold water outlets running at the same time, if you don't, the problem is the capacity (size of the pipe) of the cold water supply to your flat (providing 2 to 3Bar mains pressure), or some other flow restriction - too many elbows, service valves (very often the case, cowboys love them), blockage, etc. If I had two bathrooms, I would like to use two bathrooms independently...

Just a quick answer. I would suggest having an invented cylinder fitted to cope with the demand on the water supply. However I would perhaps put a boiler in with a better flow rate but to be honest I it will only supply the pressure from the mains. I'd also check for flow restrictors in the boiler.and possibly water governor on the system. All the previous comments are useful and I agree with them. Ideally it needs looking at first.many thanks Tom

Hi, a cold mains booster pump will only give you 12 ltrs a min max. Your boiler will give you upto 15 ltrs a minute, if your only getting 12 from the pump. That's the limit of what you can have.
You can measure the water flow rate by opening the cold tap fully at the kitchen sink and timing how long a ltr jug takes to fill, then devide it by 60 seconds to find the flow rate per minute. Typically mixer showers have between 6-12 ltrs a minute flow rate.