Rather that use horribly long and cumbersome list of decisions, you can use honorary title, with gain_effect = { } of marrying and instantly revoking itself.
My way is more convenient I think. The options only appear if you select the "show co-rulership options" decision, and can be turned off with the "close co-rulership options" decision. You can see all your options in a simple list rather than having to look for the character you want to "marry".