Global accounting giant Deloitte is advising Berwin Leighton Paisner and Bryan Cave on the structure of their proposed merger, as partners at both firms gear up to vote on the proposed combination in the coming weeks.

The Big Four accounting firm is working on the details of the tax structure of the new firm, which could go live in January, for inclusion in merger proposal documents, said a source briefed on the matter.

BLP and Bryan Cave are reportedly looking at creating a financially integrated firm, as opposed to an entity operating under the Swiss verein or company limited by guarantee structure used in many recent transatlantic tie-ups.

In the announcement confirming the talks, Bryan Cave chair Therese Pritchard said in a statement that the combined firm, “would be one of only a handful of global firms operating in a one-firm structure with more than 500 lawyers in both the U.S. and internationally.”

A partner at one of the firms, who requested anonymity when speaking about internal matters, told London-based Legal Week: “I don’t think the plan is to go for a verein—the firms want to be as financially integrated as possible.”

Fully integrated transatlantic mergers are rare, and present significant challenges due to the different tax and accounting systems used by U.K. and U.S. firms.

“The big challenge is managing different sized profit pools and compensation,” said a senior partner at a firm that has completed a transatlantic combination. “What sits behind all of that is the reach of the Internal Revenue Service. All U.K. partners will have to file with the IRS in the U.S.—and that is a huge administrative burden.”

There are also other issues that could complicate a close integration between Bryan Cave and BLP, the latter of which saw merger talks with Greenberg Traurig collapse in early 2016.

“If they are going for full financial integration, that is bold, and there will need to be a mechanism for evening out discrepancies in the profit pools in the U.K. and U.S.,” said the anonymous senior partner at a firm that recently went through such a process. “Other mergers have used measures like having three-year staged integration and specific contributions from partners to deal with legacy liability.”

It is expected that the partnership votes at Bryan Cave and BLP will take place before Christmas, said partners at each firm. One partner said that work is already being referred between both parties, noting that BLP and Bryan Cave have shared “an impressive amount” of referrals.

If the merger goes ahead, it will create a 1,500-lawyer firm with roughly $1 billion in combined gross revenue and 32 offices in 12 countries around the world. Partners at both firms have said that discussions have already been held about Bryan Cave moving its London-based lawyers into BLP’s Adelaide House headquarters if their tie-up talks are successful.

BLP, which last week saw two partners leave its ranks for Bird & Bird and Weil, Gotshal & Manges, and Deloitte declined requests for comment.