This is kind of the reason I modified how I drain the BK for my 10g batches. I used to run the first 5g into a carboy, then the next 5 into the second. I normally brew hoppy beers and wondered if the 2nd 5g would be more bitter or have more hop character than the 1st due to longer contact with the hops(probably negligible, PC chills 10g in 10 mins). I hadn't noticed a difference but being somewhat of a perfectionist, I put a tee in the drain line and now fill both carboys simultaneously. Could be the solution you're looking for.