a few more responses:
So, all kinds of things can happen to clog a 1" line like wandering snails and fish. I would tee into a 2" pipe and dial back the pump so a partial clog of one would not overflow the display. I have an overflow with teeth and still managed to get a chromis in my single 2" overflow. If it was a one inch line, the DT would have overflowed. HTH
then the last:
The problem with Durso, Stockman, and other similar standpipe designs is that they are very highly dependent on regulating airlfow to limit the siphoning effect and noise. Teeing (2) such standpipes into a single pipe only serves to complicate the interaction between the two standpipes. Using a larger (2") main pipe will limit (possibly negate) the interaction but may generate noise due to the fact that the two pipes feeding it will not fill it and there will be a large volume of air that may churn the water.
In context to your setup, it sounds like you are going to utilize (2) strainers feeding the standpipes. If that is the case, then your surface skimming is going to be limited. On the other hand, if you have two individual overflow boxes doing the surface skimming, then you are going to struggle to keep (2) Dursos balanced.
To be clear, air assisted standpipe setups are only quite over a narrow range flows based on the pipe configuration. Your results will be hit or miss based on many variables (with out without them terminating in a single large pipe).
Before you go much furhter with your setup, it may be prudent to spend some time reading about different overflow designs.