Someone in my family tree accidentally merged two accounts on geni.com, is there any way this can be undone?
Please note that a Tree merger is essentially the merging of two profiles in different Trees into one profile and Tree. Therefore, you can undo a merge only before the profile merger that connects the two tree has been completed. Once that profile merge has been completed, the trees are merged.
Please note that once you have merged your Trees it is irreversible. Therefore, it is important to pay closely examine which information you wish to keep, regarding each duplicate Profile. If the trees are already merged, you can use your Privacy and Family Group settings to manage privacy between the two sides of the family.
To reverse a merge, you will first need to add the deleted profile back to the Tree with the correct relationships. Once that is done, you can email firstname.lastname@example.org with a link to the merged profile and the incorrect relationships that remain on that profile.
On the Geni wiki I read that they're planning a "Tree split" option which would probably solve your problem.
Tree Merging is currently available. Tree splitting  will enable existing trees to split.
On the other hand it says quite clearly:
Once you have completed a merge: It cannot be undone
Source: Tree Merging