Hi Twinbrooks,
you can use the special execution parameter 'include piping input files " function which lets you combine two files, (or lets you view the two files together but not combine them). Have a look in the user guide which should be given as a PDF under the help menu.
Watch your node numbering when you do this. If you have common node numbers on the two models for different points in the piping system then when you bring the models together you get a great big mess of piping.
Only the point on both models that is the same point on the piping system should have the same node number.
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Miss Itchy