Titan's head of research Anthony Waithe defected to rival operator CBS Outdoor.
Waithe, who has left to do the same job at CBS, follows group sales director Steve Atkinson who left last week after only nine months without a job to go to.
The number of departures from Titan since it acquired Maiden last year is understood to be around 80 people - though this was not confirmed by Titan.
Such a high staff turnover at the group is fuelling speculation that Titan has failed to turn around the losses of Maiden, which had debts stacking up to £40m when it was bought this time last year.
One industry insider said that it was the "beginning of the end for Titan", which despite still owning 85% of the audience in the UK's outdoor rail market, recently lost the significant Northern Rail contract to CBS.
The main problem, say insiders, is a financial one concerning the terms of Titan's Network Rail contract, which includes significant mainline stations such as Manchester Piccadilly and Edinburgh Waverly.
Industry experts say the terms, inherited by Titan when it acquired Maiden, promised overly generous payments to poster site owners.
Titan chief executive John Slatkin conceded that the group needed to develop "a more concrete strategy" for its future.
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