This is a general comment rather than an answer, but it doesn't fit well in comment boxes...
Considerations of "boundary" between TPSE and PSE (or MSE, or MO) should be left out of definition of the scope of TPSE. In fact, it is to the advantage of all sites if the "boundary" is kept fluid.
As far as migrating posts is concerned, the rule is simple: if the post is clearly off-topic for one site but it is clearly on-topic for another site, then and only then should the post be migrated. This rule applies to all SE sites; there are no special waiting rules and such.
Based on the MO/MSE experience. There has never been a lack of experts on MSE. For example, from the current user front page on MO, around 10 seem to be somewhat active on MSE. It is likely that a similar overlap will happen between TPSE and PSE and that is good for both sites. Defining a "boundary" can only hinder this natural overlap.
Other than the scope of the sites, the only essential difference between TPSE and PSE should be the level of the answers. Answers on PSE will generally be more accessible, whereas answers on TPSE will assume a great deal of background. It is perfectly fine if the same on-topic question is asked on both sites. In fact, PSE should be the natural place to go for a more accessible version of an answer from TPSE.
This post has been migrated from (A51.SE)