The issue is that the evaporation should be about the same, so doubling the total water will actually mean doubling the evaporation estimate, which is not right.
So you should double the water less the evaporation, and then add water to counteract evaporation.
Assuming you do full volume BIAB.
Not perfectly doubling the water while doubling the grain bill will mean a slight increased loss of efficiency due to the grain absorption being at a slightly higher gravity.
So, you could counteract that... possibly try to work it out... or you could just add a bit more grain.