In case you want to split a seat, and assign the same seat to 2 guests:
Right-click on the seat you want to split
Select the "Split seat" option from the menu
The seat will be split in two halves.
Now, simply drag guests from the list and drop them on the seats.
The naming of the second seat (eg. G.1.4.bis, G.1.4.b, etc.) can be customized for each workspace. Just let us know which is you're preferred definition and we'll apply it to your case.