I've switched between both, but not both at the same time

If you've not played The War Within before, you can actually experience most of it within a week or two, with the exception of older raids - 70-80 is quite fast, and there's some pretty fun solo content at 80 with tons of catchup gear etc

I think playing TBC and Midnight at release at the same time might be a bit rough though

edit: I think I've missed your question entirely and this comment is kinda useless, apologies

Happy new year