mmmmm, not a definitive answer but here are some thoughts nonetheless.
I CAN see a disadvantage in having 1 AND 4 crossfeeds open (or for that matter 2 AND 3, or basically one crossfeed on each side of the ship).
Located in the centre of the crossfeed manifold is the "flow equalisation valve" or "flip/flop" valve as we call it. It's function is to reduce the chances of fuel imbalance due to the normal variations in boost pump output press by only allowing crossflow left to right when the diff pressure across the valve is above approx 2-3psi.
Example:
Tank to Engine feed.
Crossfeed valves 1 AND 4 open.
Combined output pressure of #1 boost pumps 15psi(old worn out units)
Combined output pressure of #4 boost pumps 20psi(new units fitted)
Result:
In addition to feeding #4 engine, the output of #4 boost pumps will overcome the "flow equalisation valve" due to the lower pressure supplied to the left side by #1 pumps and will supplement the fuel feed to #1 engine. It has a clear path as 1 and 4 crossfeeds are open.
#4 tank quantity will decrease faster than #1 = imbalance.
With only ONE crossfeed valve open across the ship, there is nowhere for the fuel to go other than keeping the crossfeed manifold pressurised up to the other three x-feed valves.
mmmmmm, Why is it number #2 crossfeed that most operators use to keep the crossfeed manifold pressurised?
As I mentioned, not definitive, but it could just be a case of physical location. The output line(fuel feed line) from #2 boost pumps is closest(in a physical sense) to the crossfeed manifold pressure switch so there's less chance of the switch going low pressure.
(For ease of maintenance,i.e quick under wing access, the switch itself is located next to one of the #1 boost pumps but it's
sensing port is right next to the #2 boost pumps outlet) Hope that didn't confuse.
