It depends on where you're at in your studies, and how well your institution supports pursuing concurrent graduate degrees. If you're just beginning your studies and you don't already have an MA behind you, then an archaeology degree could be a good way to gain extra skills that you can easily highlight on your CV for the job market.

But if you're already working on your dissertation or if adding an archaeology degree would add time and expense to your PhD studies, then I think it's probably not worth it. An MA indicates the ability to research within a field, which your PhD will already confirm. You can put any field or lab work under a separate heading on your CV, or you can simply include a material culture section in your writing samples.

I'm in a similar boat, working on a PhD in history with a lot of engagement with archaeological material. I'd love to keep doing fieldwork over the course of my career, and I hope to publish work that archaeologists read, by I don't see an archaeology MA helping toward either of these goals. As an academic historian, I don't anticipate ever being able to secure the institutional support to lead excavations, which means any fieldwork I do will basically be piggybacking on a site run by an actual archaeologist certified for digs in whatever country my research takes me to. And in publications, they should be blind reviewed anyway, so as long as I can demonstrate an ability to do archaeological research, my actual degrees don't matter.

So for my own purposes, I have a graduate certificate in archaeology—which basically just means that I took a few upper-level anthro courses and did a field school. I've worked at a few archaeological sites, mostly in my current hometown, which don't directly relate to my area of research but which have given me a much firmer grasp of archaeological fieldwork. I hope this will make my future research and writing that much stronger.

I've grouped these experiences into their own section on my CV, and it's something I could highlight for certain jobs if appropriate. But for the most part, historians tend to be very reluctant to engage with material culture, and archaeological work seems to strike many of them as irrelevant or redundant for societies which have left a written record. I could be wrong, but I fear that an archaeology MA would actually make me appear as some sort of unattractive pseudo-historian when I finally hit the job market.

TLDR: I'm all for archaeological training and for continued scholarly engagement—and I even think it could be a boon with the right spin on the job market—but at least for my own purposes, an extra archaeology degree seemed redundant and even a potential liability for my scholarly future.