Hhmm.... Actually there's an extension which comes packed with Inkscape (Extensions menu > Text > Merge). I don't know exactly what it does, and I can't figure it out just by looking at it. But by the word "merge" it sort of sounds like it might work.
But again, looking it it, it's not at all obvious what it does or how to use it. Maybe the manual.... Nope, rats, not covered in the manual.
Well, just by guessing at how to use it, it doesn't appear to take 2 separate text objects and make them into one. Although I certainly could have guessed wrong.
Maybe someone else knows?