Split Mashing 10 Gal Batch

[epml74]

I am planning a high ABV Double IPA and wanted to make a 10 Gal batch. The issue I have is my mash tun is a little small for the recipe. I'm thinking I will spit the mash between 2 separate mash tuns and then run off into the same kettle. my question is this: Do I plan the mash water volume based on two separate 5 gallon batches and then simply combine or do I take the 10 gal volume and divide it in half. Im probably over thinking this but I think my concern is that if I use Beersmith to calculate the two 5 gal batches, it will account for twice to boil loss. Any feedback is greatly appreciated.

 

[Beholder]

Plan it like a 10 gallon batch and cut the mash amounts in half, otherwise like you say, you will be off on the boil amounts.