My question is I want to see is there a way a sensor can tell if it's in parallel or in sync with another sensor? If someone was wearing on their hands or arms. It would be able to know if they are parallel to each other when the person is lifting their arms up. If one arm is moving faster then the other.
Just thought of this maybe IR light and IR sensor? Or is there a better idea?
You need to define what you mean by "in parallel" and "in sync" a lot tighter if you want primitive electronics to be able to determine that state.
For example parallel - you'd need at least 4 (actually six) proximity detectors to determine that. Put one at both elbows, and one at both wrists. Measure the distance between both pairs. Are they the 'same'? If so the arms are parallel. Well, not exactly: if they are angled differently, but symmetrical, then the distances will be the same, but they will not be parallel. Hence the need for six sensors.
This is not a trivial problem. Is this your first project? Yes or no, be prepared for a LOT of work before it works!
No idea if it'd work but maybe two gyro chips would do it.