I was thinking in your case you are better to work with alpha matted straight layers
however since I get details by bits for side-way run cycle to run cycle
to save you some work, you might consider to retime the old guy to the young guy first so the run cycle match first (same front leg with maximum extent for both – and hopefully they will have the same arm cycle when they run)
hopefully they are running over green screen on the same thread mill or something and with a locked-off camera
and maybe try to consolidate the the morph within a particular cycle and as a first pass just try to tidy the dissolve with RE:Flex
one idea is to split the morph top down in two using RE:Flex per shape morph and blend so for example if the morph works fine for left leg front but has issue with right arm front (the other cycle) then you can like that morph the leg on one half cycle and then the top on the other half cycle. It’s hard to know in advance but to be able to make a part of the morph happen real fast in part that don’t work well usually saves a lot of work.
Pierre