Fix family parameters for multivariate models in BRMS?


I have a multivariate model as such:

m1 <- brm(
  mvbind(y, z) ~ x,
  family = 'skew_normal',
  data = d

I would like to know how to force the family parameters (sigma and alpha) to be equal for both y~x and z~x. Is this possible in BRMS or do I need to go directly into STAN?

Thank you.