More likely it means that he and Miller will be available to appear on any one of the shows as needed without it having to be negotiated case-by-case like a guest spot. It's more of a "regular" arrangement in terms of how the contract is structured, in that they're available for the whole season, but with the three (or four?) shows treated as if they were one, in terms of how the actors experience it and get compensated for it. I figure it'll be a "regular-at-large" gig in a sense -- they'll be bouncing around among the different shows as needed, rather than appearing weekly in every one.
As for billing, sometimes series regulars don't appear in every episode or even get billed in every episode. Heroes worked that way -- only the actors appearing in a given episode got credited in that episode, because there was such a huge ensemble. Houdini and Doyle also does that -- actors who aren't in the episode don't get credited that week. (Which is more common with British or Canadian shows than US shows.)
