Simply because you are incapable of doing something doesn’t mean that something is unachievable.
Walking and shaking hands perfectly is as easy as it gets with the use of a root bone to position Player A to the correct spot and rotation in relation to Player B, the one you go to shake hands with.
IF you were to animatie 2 skeletons in 1 animation, would you have any sync issue at all when playing back the animation? No. Ofc. not. Same exact thing here, granted you do have to make sure both animations are fired at the correct time.
Hand shaking can further be adjusted (necessary on dynamic characters that can have different height(s)) by adding Hand IK solutions.
IK Is also a good way to fix foot sliding along with proper animations and the readily available speed warping technique.
Since you don’t seem to know the basics, Perhaps it would be best if you didn’t give people bad advice or postulate that other advice given is bad.