Here on WikiTree, the goal would be to "connect" the two trees. Since we all share one collaborative tree, the goal is to create one profile for each person.
Now, if both 'trees' are currently here on WikiTree (meaning person profiles from both sides), what you will be trying to do is merge people into existing profiles (if for instance, your maternal grandfather already has a profile here) or to build connections between the trees (such as linking your maternal grandfather to his parents if there profile is already on Wikitree).
If the data from both families is not yet here on WikiTree, the easiest way to handle the data is going to be through GEDCOM imports in another software, where duplicates are merged and relationship links built - then exported into one GEDCOM file and ran through the GEDCOMpare process here on WikiTree.
If you can provide some more information on the 'trees', where they reside, and links to any profiles if they are here on WikiTree, we can provide further assistance.