Setup should work, right? WRONG! The boiler on the right (or left, one of them) overpressurizes, and the merged pipeline doesn't get the expected 512 Steam/t!
Yeah, so if I'm understanding Greg correctly, normally any two adjacent pipes would
tick on alternate game ticks, but when you add in an extender, all three of those pipe
sections attached to it (the 2 inputs and the 1 output) tick in the same game tick
(picture a chessboard - they would either all be white or all be black). This means you
can get slightly different behaviour depending on the order in which the pipes tick.
So although you avoid the backflow issue of combining pipes directly, you can't avoid
tick-order issues entirely.
Relying on memory here, so could easily be wrong, but my instinct is that a larger
output pipe should work... the pipes over the boilers have a single output so should
attempt to output capacity/2 regardless of the content of the central output pipe
(which, if I remember correctly, is only involved in the calculation for dividing between
two or more outputs). The only reason I can think for them to output less is if there
isn't room in the central pipe.
Actually, thinking about it - that pressure valve is only going to vent once the top
section is completely full, by which point the one below it could easily have less than
512 L of space remaining in it.
For this kind of testing maybe pipe the steam into a drum? (which due to exploit-
-prevention, should just void all the steam that reaches it.)