Stata doesn't have a built-in routine for panel data simultaneous
equations. Maybe someone out there has written their own program for
this, but when I did -findit panel simultaneous- nothing came up.

If you can transform or recast your model into a non-panel framework
(e.g., by transforming into deviations from means or using explicit
dummies for observational units) you could use -reg3- to do 3-stage
least squares.

The alternative is to estimate the system equation-by-equation using
-xtreg- or -xtabond- or -xtivreg- or whatever. It won't be efficient
(in the way that a system estimator like -reg3- is) but your results
will be consistent, and will have the usual advantage over system
estimation in that if one equation is misspecified, it won't spill
over and contaminate the estimation results for the other equation.

Hope this helps.

What do you mean by not efficient?

xtsur