# Propagating RMSECV?

I have two regression models, each of which has an associated root mean squared error of cross validation (RMSECV). I would like to combine the results of the models using a weighted average to get a combined prediction. My question is how to report the accuracy of the resulting combined result: can RMSECV be propagated like any other uncertainty (e.g. adding in quadrature) or does it need to be handled differently?

I'd suggeest you directly measure the $RMSE_{CV}$ of the combined model. Error propagation of the $RMSE_{CV}$s of the submodels needs to take into account the covariance between the predictions of the submodels.