Two Firms, One Model

[kirkwilson_20]

Ok, i know this topic has been discussed on here somewhere several times before but I can't seem to locate it.
We have a model that we are developing and we want another company to add there work to the model and send it back. how can we do this with revit mep 2012 and still maintain a central model.

[sbrown]

You can't, they need to link yours into their model and you link theirs into yours.