When both taps are on in the bath, the tank in the loft empties out. There is water coming in just not fast enough. The pressure around the house is fine, and this only happens when both are running, its fine with one.
Opening the cold tap when the hot tap is running will affect the filling of the tank - but shouldn't be that noriceable. Check the isolation valve for the cold feeding the tank is fully open. Also check the ball valve, when the tank is filling (with all other cold taps off) whats the flow rate like, there may be an obstruction.
If this has only started happening lately, try fitting a new ball valve, fairly easy to do and quite cheap to buy.
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