Steady state: solution of multi-nonlinear-equation system

In the steady_state_model block, we can only write something like
But my steady state is a solution of a multi-nonlinear-equation system, e.g.
I cannot explicitly solve either x1 nor x2, therefore must solve the implicit function system simultaneously for numerical solution. How can I do this in dynare? If need external m files, how can I incorporate that into the dynare so that when the parameters change, it will adapt accordingly and that I can also do the estimation?

You could take a look at example3.mod in the Dynare examples folder. It shows how to use an auxiliary file to call a solver.