[Lucky]

The DME doesn't know (or care) if the fan is already running. All it does is monitor temps and energize a fan relay when its logic says coolant temp exceeds a programmed threshold.

You have two ways to attack this:

1) the aforementioned hardwired switch -- although I would suggest you not run the full amperage through the switch and instead have your switch control the fan relay (the same one the DME is energizing), or

2) use an adjustable temperature switch to control the fan relay -- this way it is automatic.

Or an even easier approach is to simply run your AC when it is hot. This keeps both you and your engine cooler, since the AC condenser is mounted in front of the passenger side radiator and that side's cooling fan runs continuously while the AC is on. I find this drops the indicated temp on the dash gauge by about 5-10 degrees on a hot day (from just right of the "0" in 180 to the middle-right of the "8").

Of course, YMMV.