In theory this wont matter if you can somehow blend the runnings or preboil wort. (is this All Grain?) You want to avoid a big beer in one pot and smaller in another(I assume.) But you could also use this to your advantage. I mash and boil 10 gallons but my fermentors are 6 gallon. Almost every time, I use two different yeasts for fun. I can also dry hop or keg hop differently too. Historically people have purposely made different runnings into different strength beers.
If you are going to blend it-pretned its all 10 gallons. Otherwise pretend it is two unique brews.