I'd say yes. I can't find anything that says you cannot enact both rituals or that one cancels out the other or can't be used for any other purpose.
As long as you maintain the upkeep for the daily mask contemplation for Scarred Witch Doctor and uphold the requirements of a Signifier's bonded mask, I'd say it's completely legal.
As the easiest way to play the scarred witchdoctor's mask is to play it as a bonded object* it is covered by the rules to combine both.
*the rules text about the mask is very unclear, leaving a lot of questions open. Most ob those questions are answered if you handle the mask as a bonded object instead of a familiar.
i thought you could simply take the signifier mask as your scarred witch doctor familiar via that replacement ritual for when it (the witch doctor mask) gets broken.