If you think about it, when cousins marry, they have joined two branches of the same family tree.
So the FH Diagram reflects that by showing both branches.
Presumably, you are getting coloured ribbons linking the people who exist in both branches?
I’m not sure what you mean by linking to the shared grandparents.
You need to select one cousin, and when adding their spouse, select the other existing cousin.
The grandparents should not be involved in the process, but will become common shared ancestors.
How the Diagram is displayed depends on who you choose as the Starting Person for the root of the Diagram.
Sometimes only one branch is displayed initially, but if both are shown then one can easily be hidden.
Also, there are options to adjust the coloured ribbons. See Diagram > Options > General tab Check for duplicates & Options button.
It is difficult to give precise advice without seeing your Diagram.
Unfortunately, screenshots in this Email forum must be reviewed manually by administrators, so may take some time to get through.
The FHUG Forum https://www.fhug.org.uk/forum/ is much faster.
My first post! Hi everyone.
My Family Historian 7 seems to be having a problem coping with first cousins marrying. As soon as I try and link to the shared grandparents, it keeps giving me two duplicated lines of descent. I have a James Edmonds marrying an Ethel Parker. James Edmonds mother was Elizabeth Burr. Ethel parkers mother was Emma Burr. The parents of Elizabeth and Emma Burr are one of the same - Jonathon Burr. i've completly started from scrtach on numerous occasions, but with always the same result. Unfortunately, it really messes up the tree. Hope this makes sense! Any help you can give me would be much appreciated. Many thanks.