There is an easier way to get yagi distance than actually using yagis in an array. That way is to make a collinear array of omni vertical elements. By stacking a number of omni antennas one on top of another and connecting them correctly, you can squash the radiation pattern in the vertical dimension and force more gain to the horizon. How this is done depends on what frequency you are working at. I've done this at 6 GHz using a pcb version of a series connected array of vertical half wave elements. Here is a link to another version at 2.4GHz using coaxial design that uses the same principles of series feeding half wave elements: **broken link removed**
The difficulty with the idea of using six yagis is that it is mechanically a bit of a beast, but also that you have to somehow connect all of the six feedlines in a way that power is split evenly to all six antennas while remaining in phase at each, and while delivering a good impedance match to the transmitter/receiver. That takes a bit of work.