It's usually an awful lot of extra work to manage two overlapping trees. If these trees contain overlapping information, my recommendation is to just maintain one. You'll save yourself a lot of headaches.
To create a duplicate tree in FTM for windows (possibly the same on the mac), you go to file|export and filter the exported data to include the lines you want. The file then created can be imported into FTM and be the basis of a new tree, locally or synchronized with Ancestry.com. Hope that helps.