Well, Sprint did just recently announce that they are now going to unlock their phones and network. See Sprint Nextel Agrees to Unlock Phones for more details.
Now, as for 2 lines per phone, I know that Nextel has always allowed this on their iDEN phones. It started out with their i1000, then i1000+, and continued on up to the merger. As far as I know, all current iDEN-only models support 2 lines (if you opt to use a second line). I don't know about any of the PowerSource nor CDMA-only phones.
So, if you desperately need a 2-line CDMA-only phone, you might inquire about purchasing an unlocked CDMA phone that is capable of offering 2 lines of service, and then grafting that onto Sprint's network. Otherwise, buy an iDEN-only and don't worry about it.
In addition to Nextel iDEN phones having two-line capability, the billing system is designed to be set up so that Nextel will send the bill for the work line to your employer's address and the bill for your personal line to your house.
Two lines in one phone, with separate billing - you can't beat that.
OMG What an indescribeable nightmare putting two lines on my Nextel Phone.(i335). I will spare you the gory details but it was the worst CS relation I have had to date. And it is NOT over. I have the two lines but, they are trying to convince me now that I may not call out from the second line. That the phones are not designed to do it..... BS BS BS BS BS BS BS. Tell that to the i1000 I had back in 1998 and the i90c I had in 2002. I have talked to so many CS reps including escalations and tier 2 tech support. Even the guy who got the two lines (incoming -one out) doesn't know what the **** he's doing. He is the one being told by others that it only receives and doesn't make calls on line two. Furthermore, I can't use a shared plan for some odd reason but, hey he said he will look into it we will see....