Thank you for writing to us.
Unfortunately at this time there is no way to add complex relationships like two cousins marrying each other online.
The only option available is to add their information twice.
We do hope to have this feature in the very near future. It is one of the thing being worked on as we speak!
If you wanted to work in the free downloadable Family Tree Builder Software, I could let you know how to manage such a situation.
Esther /MyHeritage Team
I would be interested in finding out how to do that in the downloadable software, and also informed when you have this feature on line.