Publicis, Omnicom seek to resolve leadership spat over CFO job - sources

* Pitfalls of 'merger of equals' emerge amid clash over CFO

* Omnicom, Publicis tie-up delayed by antitrust, tax reviews

* Both sides remain committed to deal - sources

* Dutch tax authorities cool on UK tax residency plan - sources

By Leila Abboud and Tom Bergin

PARIS/LONDON, April 27 (Reuters) - The chief executives of advertising companies Publicis and Omnicom Group are working together to resolve a seven-month-old struggle over who will be chief financial officer of their combined group if the $35 billion merger is completed, three people close to the deal said on Sunday.

John Wren, the head of New York-based Omnicom, and Maurice Levy, his opposite number at Paris-based Publicis, are in regular contact to try to settle the CFO choice, which has fuelled tensions between the two sides since September as they seek to secure regulatory approvals for the blockbuster deal, the people said.

Both remain committed to the tie-up, which would create the world's biggest ad agency ahead of current leader WPP, added the sources.

The infighting over the CFO shows the pitfalls of trying to engineer a "merger of equals" as the deal was billed when it was announced to much fanfare in July.

Some analysts, investors and rivals have expressed doubts over whether executives and staff in the two companies, especially their veteran CEOs, will be able to effectively work together if the deal is completed. Other mergers with similar profiles, including a 2006 Franco-American tie-up between telecom equipment makers Alcatel and Lucent Technologies, have foundered over culture clashes.

The CEO of rival WPP, Martin Sorrell, said on Friday that most people he was speaking to said there was now a third to a half chance that the deal would not be completed.

The tensions between the top executives of the two companies were first reported by the Wall Street Journal on Friday.

The merger calls for a 50-50 ownership split of the equity in the new company, Publicis Omnicom Group, with Wren and Levy serving as co-CEOs for 30 months from the closing.

The two companies initially aimed to close the deal in the first quarter of this year, but an ongoing China antitrust review and tax jurisdiction issues that were disclosed by Omnicom last Tuesday have delayed completion. Publicis said last week that the deal would close in the third quarter, but Omnicom's Wren now declines to predict timing.

Omnicom wants its CFO, Randall Weisenburger, to get the top finance job, while Publicis wants its CFO, Jean-Michel Etienne, the people said.

PROFIT MARGIN QUESTION

Behind the spat over the group's finance chief is a deeper question about how the new operation should be run, said two of the people.