What effect does using two different forms of solder mask on the same PWB have?
Recognizing that there are a lot of assumptions here about the material specifications involved...

I spoke with two of our partner factories and they both said the same thing:


This is typically a result of rework, where the original soldermask was partially removed for some reason and needed to be reapplied post-hoc.
Should still meet spec -- meaning, there are no anticipated concerns as a result of the mixed process. Some overlap between the masked areas was suggested to ensure that you maintain complete coverage over the full temperature range.
If needed for a "special" application (life/safety critical, aerospace, etc) then recertification/testing is required.