Batch the two two numbers? That would allow you to text them both at the same time, but I think you want to combine an old conversation from an older number with a new one? I dont think that is possible on any messaging app. I have a few friends that I text both their work and personal cells...
As for Go SMS Pro, what do you think about it? I just got it today and and going between both Handcent and Go...both have some things i love about them Go definitely seems FASTER to load but for some reason i can not get the LED to flash when the phone is charging...come on people, I have an EVO that thing is ALWAYS CHARGING!