My best suggestion for both of you is READ READ READ READ READ. There are LOTS of posts about this.
Ceoli recently had a thread about being third and how painful and hard that can be and Mono has TONS of writing about how well he's been cared for, loved and treated "as a third".
Check those two posters threads first then search.
are key ingredients. If you want to add someone you have to be willing to prioritize THEIR needs/feelings/desires, not just your own.
"Love As Thou Wilt"