Shower pressure drop

[Schugeorge67]

I have two showers in my house. ALL heat and hot domestic water is provided by oil fired boiler Only one shower can be used at a time or the pressure drops to a trickle. In fact if one is in use and hot water is turned ANYWHERE else in the house the same thing happens. There is a mixing valve in the system as well as a circ pump. Finally, the overalll pressure is slighter better in cold supply than hot water. Any ideas?
Thank you

 

[ShaunCorbs]

What’s your incoming water pressure ?
with one outlet open ?
What’s your incoming water flow rate ?
With one outlet open ?