If the two sources came from merging entries in FTM databases, I would be inclined to leave them alone. There must be some reason that FTM chose to make two sources.
As for merging sources, I have not found a place where sources can be merged. Please advise where you find that.
However, I do find where, in the Source Workspace, you can select a source, right click, Manage Sources, Replace and then follow a string of screens where you eventually replace the original source you selected with another source. This is different than a merge because the replaced source info is extinguished and replaced with the second source (not info from the citations - but info in the source itself, like repository, author, publisher, etc.).