I can run the scenario in ElectricalOM for you, if you can share the details of the supplies, conductors and protective devices. I would even be happy to do it on a teams call if you like.

Only would take a short time (no pun intended)

I've built a few systems with multiple sources and I don't think it is possible to have two sources active at the same time. Why do you need both sources to be active at the same time? I know in reality they may be but for carrying out the calculations do you need too.

It is a limitation of the design of the software.

In that scenario, if you do need them to operate in parallel, I would likely add a third virtual source "1+2" with the characteristics calculated to be the equivalent of the two sources. For everything downstream of the MDP panel you can run the calculation with just this supply active and all bus couplers closed. For the supply cable sizing and incomer settings you'll need to model individually of course; depending on load balancing across the MDP sections and operational scenarios that might mean a separate calculation.

(Of course while this would account for the increased fault current and load capacity, this approach would not model circulating currents or load sharing across the supplies should they not be identical; if that's relevant you're far beyond the software and it's pen and paper time)