How to find the IRR for a cash+share rollup acquisition?

Hey PE monkeys,

LBOs and acquisitions are not really my forte, so I'm trying to figure out the best way to model this situation and am a little confused. 

My PE firm owns PortCo A. They are intending to acquire PortCo B (competitor in the same industry). They are intending to offer the owners of PortCo B a mixture of cash and shares, such that the owners of PortCo B end up with a 10% stake in the combined company A+B. 

My question is how do I think about the IRR of the acquisition of PortCo B? If it were a cash acquisition I could just model it out fine and consider it a standalone company (i.e. exclude synergies) for returns purposes. But the fact that I am paying for it with shares in PortCo A makes me unsure of how I should consider the IRR of the standalone deal. I am effectively paying for PortCo B partially using forgone cashflow at exit of PortCo A. 

Of course I can figure out the blended IRR across both deals - use the 2 cash offers and model out the combined exit. But what do I tell people when they are asking for the "standalone IRR" of the PortCo B deal?

Sorry if obvious question. What do you guys do at your shops?

I'm starting to think looking at standalone IRR isn't even the right question in this case. Maybe you'd look at blended IRR and just compare with your original PortCo A standalone underwriting. Or maybe you'd look at some kind of accretion dilution since this is effectively a merger?