Still one has to check both sites regularly, or avid users will slowly drift away, toward this new proposal?
Over time, I would expect that people will gravitate towards the Mathematica-specific site. There might still be some questions on the other sites, but the regular users will mainly be in the new site. Consolidating on a single site instead of splitting attention across multiple sites was one of the main rationales for the proposal.
There is a feed of all questions tagged mathematica across all sites that flows into this StackOverflow chat room, if you want to check for questions across all the relevant sites. Occasionally someone posts a Mathematica-related question on math.SE or SuperUser.
I can see two extreme measures that could be taken.
Move all posts tagged
mathematica to the new site as a means of seeding it. Should not before open beta, though.
Migrate all new questions on SO tagged
mathematica over to the new site by default (wether automatically or by moderator intervention).
Any of the two can be done independently of the other.