You do not demand. You request.
See what kind(s) of open model relationship
you guys are up for and agree on.
Figure out what you want and what you need and what your limits are. Tell them
what your wants, needs, and limits are. Ask for theirs. Then respect their wants, needs, and limits.
Be careful of common pitfalls.
Here's some resources:
Talk about how you want to be should the dating experiment end and people break up. Go slow. Especially since you cohabitate -- nobody needs to be losing their home life just because you decide to date. If you break up... will you be able to secure a new home in a decent amount of time? Or will this threaten your financial security/ability to live ok?