Okay to combine sealant brands on same faceup?

On darker resin dolls I use Citadel Munitorum Varnish because it doesn't leave a white cast like MSC can (especially after multiple layers of it), but since it's got a more satin finish my final sprays are with MSC. No problems here!

 

I've used msc on top of liquitex airbrush sealant with no issue. That is a little bit different though.

 

I think it should be fine as long as all layers are fully dry before you do the swap. I wouldn't personally mix two spray sealants together with them both being wet (like, doing part of a layer with MSC and finishing with ZM) since you don't know how they might interact with each other as they dried.

 

I’ve used ZM spray over MSC (high humidity day and my layers were disintegrating with MSC; ZM spray is hardier under those conditions) with no issue.

 

You already got an answer, but you might benefit from this link (it answers such questions, and many more):
THE BIG BJD FAQ

 

It doesn't hurt to do a quick google to check if the brands you're using clash, I know the sealants FAQ on here mentions some brands react negatively to each other! (/threads/all-about-sealants.605416/)