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I would like to simulate two kinds of shock in the same DSGE model. The first shock aims at generating uncertainty between four kind of policies (therefore a discrete random variable) and the second shock is simply from a normal distribution. I know how to simulate a single non-normal shock: viewtopic.php?f=1&t=1855

However, I am finding troubles understanding how to make these two kinds of shocks coexist into the dynare simult() function. Should I treat them separately until the random generating process and then stack them into the vector DynareResults.exo_simul?

First of all, see the caveat that this approach is only valid at first order due to certainty equivalence in
http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=7257

Second, what you need to do is change those shocks that are discrete in DynareResults.exo_simul while leaving the other ones unaffected. This requires your covariance matrix to be diagonal as otherwise there would be interactions. If there are no interactions, you can just change them one by one.