Usually I would wait til I feel that the other person is relaxed and knows me enough ask the appropriate questions)
Too early and they think your looking to settle Down, then panic and leave.
To late and they think you are hiding something.
So I usually wait for the, 'where is this going ?' Conversation.
It's also a common mis conception that someone withMH issues will understand you better, if you also have MI. I actually find them to be far more fixed. That their version, symptoms, methods of dealing are the correct ones.
Even if This isn't the case, often a crash can happen at the same time.
Subconsciously bringing each other down, mis reading each other, all the same things that apply in a normal relationship only the fallout can be much worse.

All relationships have these pit falls though. I just don't think 2 people with MI will automatically understand each other better.