while I found that brm- multi-variate models can estimate two outcomes based on the same data/model (with different parameter estimates by each outcome, and correlated parameter estimates), my search in the net and vignettes did not yet reveal, whether brm also allows to -directly- estimate one and the same parameter (instead of simply correlated) for two (mutli-variate) outcomes. For example:
brm( bf(DV1~ 1+ b1/2, DV2~b1, b1~1, nl=TRUE), priors etc)
such that I get only 1 estimate for b1?
This is usually referred to as ‘common cause’ mechanism and is brilliant to ‘fix’ the model on additional data (reducing its flexibility in finding solutions). It should be part pf brms! :)
If not, I guess it might work to do it in a multi-variate fashion and doing some tricky parameter definitions in two different models and then fix their correlation at 1 somehow?