I'm confused about your question. You can have two numbers, and when you select a contact to text for example, if you check both numbers it's one contact name with a (2) next to it which, of course, signifies the numbers checked to text the contact with.
I believe Handcent will do what you want. When I text two different people the same message, it keeps it on one thread. Should be no different if two numbers but same person.
I see the need . I have 2 mobile phones and no time to tell all my friends/family which one to text at any given time because I'm using one or the other. It's easier for them to text both. On my Omnia I had a text forwarding app so if I was using my other line I just forwarded everything to it. Dumbphones don't have that ability though.
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