On this siphon, I have it all drawn out before construction. Why are we using a T and two intakes for the siphon stand pipe?
Theories on this: One pipe can become clogged with media or roots, and make starting/stopping less consistent and risking overflowing. So by have two, it functions as a back up, but being that they flow into the same single riser, it will not add more flow rate but will decrease intake pressure as split between two pipes. Therefore reducing probability of trouble later on.

Why two 45's instead of one 90 on the inside? Two 45's will reduce resistence on flow rate, but a 90 is more compact. With each barrel 1/2 being 11 1/2 inches; once we account for the two 45's and having to be 1 1/2 inches below edge of barrel, the stand pipe is what....8 inches or so tall. This isn't going to be very big.

Then once outside the barrel when connecting to 3 inch main drain line, is a 90 downward turn okay? Or another 45? A 90 at the end would cause more back pressure and resistance, but would direct the water more downward. If I take a hole saw and cut say 1 1/2 or 2 inch hole in 3" main in the top, the 3/4 90 could be fitted slightly to the inside of the hole to prevent back splash or overflowing on outside of the pipe. This doesn't need to be mated or anything, and the extra air gap would allow for more gas exchange but also prevent a pressurized line, burping, etc.

Thoughts on this?