Multirotor with horizontal motors

For an experiment I want to build a Octocopter with 4 Horizontal motors and 4 vertical motors. The 4 horizontal motors are expected to be used for XY plane stabilization and XY movements. The target is to move the drone in XY plane without any tilt. Can someone please suggest me a starting place to begin with for developing such a custom configuration ? Can we achieve this through custom motor mixing ? Or do we need to write a new control algorithm ?

The code already supports this, please look at the the 6DOF multirotor code inside arducopter.

6DOF using 6 motors.
Arducopter 6DOF