My 125 has two built in overflows. Am I asking for trouble if I connect those to one drain line using a tee? It would be easier to plumb the tank this way for the filtration. I will be running a 2200 gph pump.
If you connect two 1in lines into a Tee fitting that has 1in on all sides, then you have effectively reduced the amount of flow to the capacity of a single 1in drain line.
You either need to run the drain lines completely separate or use a Tee fitting that has two 1in fittings and a larger (maybe 1.5in) connection for the drain side. You would then run larger pipe from the Tee fitting down. To make this type of Tee fitting, just use a 1.5in PVC Tee and get two reducers to drop the two sides down to 1in.