Part of the reason why T-mobile and Sprint have fought so passionately over the matter of the proposed merger between the two of them, was for the capability to be able to compete on the same level as the two largest wireless carriers in the U.S., Verizon Wireless and AT&T Wireless, for spectrum in the upcoming auction as well as overall service. While the merger still has no confirmation from the regulators on whether or not they'll approve the deal it seems that Sprint and T-Mobile's merger plans aren't the only plans that the two competitors have in the works.
It would appear that they have put together a separate deal to band together(no pun intended)and compete in the upcoming auction for the 600MHz spectrum bands. With the deal being worked on completely separate from their other plans, it's a wonder if both T-Mobile and Sprint had both been in talks over this second deal for a longer period of time than they have been discussing the merger. The rumors state that Sprint and Big Magenta will follow through with the joint spectrum buying process whether they succeed in getting the merger approval or not, and that they're both trying to raise a rather large sum of funding to allow them to place bids on TV broadcast owned spectrum. That sum amounts to a hefty amount of $10 billion, although no confirmation has been given on if that number is actually true.
Whether T-Mobile and Sprint's plans to join forces are so they can keep as much spectrum away from the other two larger carriers as they can or just to make sure that they get the actual needed amounts to progress themselves is up in the air and something we may never know for sure. In addition to the possible joint purchase considering they can even raise the funds to bid, both T-mobile and Sprint may be looking at a long process for the merger to get approved so it's possible that they are looking at this deal as a contingency. In any case, both carriers seem to be doing all they can to assure some footing on equal ground with Verizon and AT&T.