If you think about it from a purely operational engineering point of view the ONLY reason to have 2 lines is to bypass the general one. I realize that it also seems to be an ego thing, but if you really wanted to look at this from a mathematical point of view, you would use only one line in strict order of the BG with the extra provision that once the currently boarding group number was greater than yours, you could use the bypass. You could also make a small optimization by having the very first boarding group initially line up in the bypass line while the second boarding group lined up in the general line and start by emptying the bypass line first. Now here is the amazing thing about this optimization - it is almost EXACTLY what the old UA boarding process did! How about that - there actually was a real engineering based reason why the old UA boarding process worked better than anything they have invented since with their bizarre attempts to emulate Lake Woboggon's every passenger is above average.