I agree, I would put a "T" in, but I would also move the pump all the way to the right side of the sump and have only one overflow pipe going into the left side of the sump.
Having the overflow pipe split like that is afforing you no benefit. If anything it's creating more bubbles by having them come in on both sides of the sump.
Just cut the right over flow about 6 inches from the "t" (under the union) and cap it. Then cut the left overflow pipe just before the last downward elbow and place a "t" there.
From that point, just drop the overflow water out of the pipe, do not put a ball valve there. God forbid a snail or something got washed down there. You want it to flow right out.
Finally, extend the pipe above the "t" you just added (about 6 inches) and cap it.
Be sure to drill a small 1/4" hole in the cap to help reduce the noise from the pipe.
That's what I would do.
Good luck!