The group has issued a tender for the separate accounts. Both franchises are looking for integrated agencies to develop aboveand below-the-line activity, with an increased focus on digital marketing, to raise awareness of the two brands and revenues.
Each agency will be responsible for providing creative and brand strategy and planning.
Southeastern has previously worked with Rapier, which created the "Great days out" campaign, which featured pictures of famous landmarks and attractions in close proximity to people's houses, including on their doorsteps.
Southern, part of Go-Ahead, took over the Gatwick Express franchise last year from National Express. The franchise, which runs between Gatwick and London Victoria, previously worked with Black Cat, now part of JWT.
The business is separate from the Southern Trains brief that was won by VCCP in August.