I'm someone who works with the Celtic pantheon, but recently have found myself being called pretty intensely to Bast. I've heard that mixing pantheons can be either incredibly disrespectful or just fine, but I'd rather see what you guys have to say about it. I don't want to disrespect anyone! :<
Late response is late. Please keep in mind that two of the three of us are Abrahamic; you said "you guys" so I let the ask sit for a while to see if they had opinions, but anyway, it's been a bit so I'll just fill this in.
I (Mod L) am a Gaulish polytheist. (I have no idea how common knowledge that is since we do most of our talking on unrelated things, so whoops let's reiterate.) So I'm the closest thing you'll get to a "Celtic authority" on this blog idek.
Here are the guidelines that I would use, and would probably be supported by other polytheists in the comm:
Respect each god as an individual. Don't try to lump them in with each other.
Research each god and how they relate to other gods. For example, I believe (and Mod D can call me on this if I don't remember) Judaism is henotheistic; you can accept the existence of other gods, but YHWH must be held above all of them. Similarly, some lwa must be housed above/below/etc other lwa.
Be sure that these two gods can, in fact, be compatible. If, for example, you're following X, and X requires that you be a pacifist, then you can't follow Y, a god of war. That kind of thing.
Give each god their due diligence. Don't say, "well, since this offering worked for X, I'll give it to Y too." Individualize. Maybe give them separate altars/devotional spaces.
That's more general. As for your specific case:
Not entirely sure what you mean by the Celtic pantheon (there is no Celtic pantheon; "Celtic" encompasses a variety of cultures and religious paths, from Gaelic to Welsh to Gaulish), but most people typically mean Gaelic, so I'll operate off of that assumption for the most part. Like I said, I'm not Gaelic, but I'd recommend chatting up echtrai and/or looking up some of the people on her listings page. They might have more to offer on that front.
As for Bast, the only Kemetic polytheist I know of who for sure is a gr9 resource is satsekhem, so you should totally do that too.