There isn’t really a way to merge two people in different trees online. What you can do is merge two family trees. If you are really interested in trying to do that, let me just tell you that is a little complicated for online trees.
Firstly, you will have to export both your family trees to Family Tree Builder. Then you can use the export branch feature and export a specific branch from one family tree and then import in to another tree in the Family Tree Builder software.
To do this, go to Family Tree Builder and import the first GEDCOM file (or open your existing family tree project if this is your basis for the merge). Then import the second GEDCOM file, using the ‘File>Import Gedcom’ option in Family Tree Builder. A wizard will open, asking you to select the GEDCOM you wish to import. In the ‘Configuration’ page in this wizard, there is an ‘Output’ section, which gives you an option to merge the new GEDCOM file into the previous project. Choose the Merge option and complete the wizard. Please note, that if there are duplicate people in the two GEDCOM files, they will remain duplicates and will not be consolidated, you will need to delete the duplicates manually. We intend to provide more advanced features for merging GEDCOM files and consolidating duplicates, in the future.
Next time that you have a technical question, please write on our support message boards :)
If you need further assistance. I would be happy to provide it to you.