Many years ago, central cleaning system installers used a fitting like this…
Can you see why this might be a problem? If a piece of dirt or debris that is similar to a popsicle stick, toothpick or twig gets into the system, it can not make the turn toward the power unit and will actually cross over to the other truck line. It then will go back and forth as the two trunk lines are used and eventually gather other debris and clog the system.
If you see this fitting as part of your tubing system and have suffered from clogs, now you know why. The recommendation here is to have a full-time vacuum expert remove that fitting and replace with either a sweep tee or a tee wye as shown here.