Hi Pieter,
You cannot currently embed on workflow within another, although that has been suggested as a nice extension to Galaxy.
You can for instance start a new history, run workflow #1 with N tool steps, then workflow #2 with M tool steps, and save this history as a new workflow #3 with N+M tool steps. This has the downside that if either workflow #1 or #2 is changed, this does not alter #3 - which is an independent copy.
Peter