There are at least a few problems with that happening from what I'm aware.
1) That would require Verizon's cooperation and it isn't the company under scrutiny. For all Verizon cares AT&T's merger can fail and those guys can get stuffed with the breakup bill.
2) Verizon and AT&T use almost completely different LTE frequencies. Although they are both in the 700mhz band, they have separate subsets between 700mhz and 800mhz that don't overlap much.
3) Verizon and AT&T currently only use LTE for data. Voice is still carried over the existing 3G network until some future date. Even then, whose network would these hybrid phones be designed to fall back on in the absence of LTE? All the hardware to support multiple network standards isn't free in terms of financial cost or design constraints.
4) If the US government really wanted spectrum to be shared in this way it would have enforced it when it freed up the 700mhz spectrum a few years ago instead of auctioning bits of it off to individual companies for virtually exclusive use.
Absolutely. Device portability would add real competition to the market, but in reality, AT&T would never agree to that because their whole rationale for the merger is to reduce competition (per the leaked memo).
Verizon would also take major issue with this and most likely not allow any such AT&T devices on their network... and if VZW did certify a device, they would only permit the most crippled hardware from AT&T on their network.
True competition between the two telco giants is an exciting proposition though, hopefully this is eventually accomplished. Just look at what it did to the long-distance market: http://video.wttw.com/video/1949293907/ (this is a PBS documentary on MCI breaking up AT&T premiering this weekend--good timing!)
