It's not as if Nasdaq OMX Group Inc. and the IntercontinentalExchange Inc.'s interloper bid for NYSE Euronext doesn't already have its share of regulatory challenges, but the unprecedented structure of the deal adds even more uncertainty to the mix.

Specifically, there's some concern over the fact that any Nasdaq-ICE joint bid for NYSE will require three shareholder votes for completion. According to several dealmakers, nothing like that has been tried before, not to mention that this may be the first time two public companies have teamed up to make an unsolicited bid.

"There are a lot of firsts here," says a source.

Adds another, "The deal makes a lot of sense for the companies, but the question is, can you get the shareholders to agree to it?"

Nasdaq's bid is worth $42.50 a share. Of that, $14.24 is cash, and the rest consists of 0.4069 Nasdaq shares and 0.1436 ICE shares for each NYSE share. In theory, that would trump Deutsche Börse AG's all-stock offer of $35 a share.

While NYSE has offered no indication that it is open to the deal, contemplating the potential of a transaction leaves the head spinning. Both deals offer a fair share of uncertainty. European competition regulators have already signaled they'll be scrutinizing DB's bid, while politicians concerned about a perceived foreign takeover of a U.S. financial services icon have fired warning shots. For their part, Nasdaq and ICE have tried to bring Washington to their side by noting the domestic nature of their bid, even as they acknowledge the antitrust issues likely to be raised by the Department of Justice. "Just the mention of a Nasdaq-NYSE merger sounds anticompetitive," says one arb.

New York Sen. Charles Schumer has already thrown some cold water on prospects for a winning bid by Nasdaq. He said on April 1 that he was worried how a merger would affect jobs in New York City.

But even leaving those issues aside, the concern about any three-way transaction for NYSE is that it remains vulnerable to collapse, a fact that is certain to be noted by NYSE's negotiators if the deal ever moves beyond the proposal stage. That would likely entail a high breakup fee, similar in scale, if not larger, than the €250 million ($358 million) fee negotiated by Deutsche Börse in the event NYSE is unable to complete their deal. "The NYSE wouldn't want to take on that much risk," says one source. "It will definitely be something they use to push back against any interlopers."

Shareholder votes tend to expose deals to uncertain outcomes. Take the failure in November of Blackstone Group LP's $605 million bid for energy company Dynegy Inc. The deal collapsed after shareholders, led by Seneca Capital Investments LP and Carl Icahn, rejected the bid. Seneca also rejected a $665 million bid by Icahn for Dynegy, which prompted the company's management team to resign and left the future of the company in doubt.

Keeping shareholders in line in one deal can be a challenge, but multiplying that by three introduces many challenges for the dealmakers involved in negotiations. Consider the hypothetical possibility that one bidder in the transaction rejects the deal, even if the other bidder and NYSE accept it. Who would be responsible for any breakup fee?

"The bidders would have to come up with their own interparty agreement," says one source, noting that the agreement would have to cover the potential for negative shareholder votes, regulatory risk and the possibility that any one party to the deal is itself jumped in the transaction by rivals.

One source notes that it's not uncommon for bidders in such large-scale deals to leave themselves open to takeover bids. It's not difficult to contemplate offers for Nasdaq, including any one of a number of exchanges in Europe or Asia looking to expand. "The bid puts [Nasdaq] out there as open to making a transformative play," says a source.

One mergers lawyer not on the deal says that the simple fact that so many vulnerabilities exist makes it likely that the transaction will be targeted by activists, which has the effect of increasing the vulnerabilities. "When you start adding it all up, you quickly get to a point where it becomes a self-fulfilling prophecy," the lawyer says.