2 5 gal Extract Kits = 1 10 gal Extract Kit?

I introduced my in-laws to extract homebrewing a while ago and they've really taken a liking to it. They are solely doing 10 gal extract batches by ordering two of the same kit and brewing it all together. I think ingredient amounts are not a linear function of batch volume, so I think they are not making the beers as intended. i.e. not enough/too much grains, hops etc. I'm sure the beers turn out great as they will testify, but not as accurate to what they are supposed to be.

Is this the case? If so, how do you calculate a 10 gal batch's ingredient amounts from a 5 gal recipe? Thanks.

I'm pretty sure extract brewing is linear - to make a 10 gallon batch from a 5 gallon recipe just double up on everything.

Of course, things, like hops utilization, change if your boil volume does not double

As long as they double the boil volume for your double batch they should be fine.

If they do 10G batch with the same partial boil as a 5G batch then the hop utilization will be way off. It will still be beer, however... just extra malty.

Ok, thanks guys. They do a full "10 gal" boil, but I don't know how much extra they add for boil-off. Hop utilization was what I was thinking of most.