No worries,

Once you have built the new model, enter the 'master' piping input document that you want to contain both docs.
Under 'Environment' you should find 'Include Piping Input Files'.

The help files should be good enough to get you on the way with this, however you may want to test it with some simple models just so that you have familiarity with it. Otherwise, there are plenty of posts on said subject.

Cheers