Actors who are recurring can do that. They are NOT on contract. Joe Mascolo (Stefano) WAS on contract.
I also think, since Days films so far ahead, it is possible to see some actor on two soaps, as he other shows are not that far in advance. i.e. Actor A can be on contract for 3 months at Days, film a lot of scenes, then go to another show, film some scenes that are seen even before the ones at Days.
Actor B can be recurring at Days, film a bunch of stuff, not seen for 5 or 6 mos., can be recurring elsewhere (Wally Kurth/Justin does this) film there, it is seen way before the Days stuff. Or be recurring at Days, film scenes, then go on contract elsewhere, so one sees the actor there, Days stuff not even shown yet.
Just another consequence of the show filming so far in advance. As noted, they took a month off last March, 2 months Nov/December, and are still 6 mos ahead. That is absolutely ridiculous.