I think in the short term you could use call forwarding to the phone you were going to use at that particular time.
I have roughly 1400 song words in my phone and mirror my files into my car mobile just to be sure to be sure …. different size screens for reading

I have never heard of anyone wanting the same number on two or more phones although some phones can have two numbers/sim cards.

If there was a demand for it im sure the phone companies would be working on it although security probs could put them off if anyone other than the News of the World was listening to your conversations as well as looking at your texts :<) Smiley