Tom and Jamie,
Waste spark still has the same number of sparks per revolution, in the case of a 4 banger, there will be 2 dwells per engine rev, yielding 4 sparks per cycle (720 degrees). The coils alternate dwell cycles. 1 and 4 get one coil, 2 and 3 get the other.
Now for the fun part...
Provided that the power feed to the coil unit is in the current transformer path,
at low RPM, the tach will in fact work to the same extent that these tachs work with any other electronic ignition.
The trouble however is how the tach responds if and when the two coil drivers are on at the same time.
On a wasted spark engine, this occurs at higher RPMs as the dwell% increases and the two dwell phases begin to overlap.
When the two phases overlap, the currect will shoot from say 4 amps to 8 amps. This may or may not trigger the tachs current sense when this occurs.
Very likely you will experience an anomaly when the RPMs rise into this regime.
In my experience, I ran a basic 4 post coil, which required a 3ms dwell time.
Each coil can get up to 10ms (100% duty at 6000rpm), The 2 coil dwells will begin to overlap at 50% duty cycle, which would have been around 10,000rpm in my case. This depends on the RPM the engine will run to and the specific dwell time that the coil needs for adequate spark.
In my case, the tach worked and did not hit the point where it was an issue, though the RPM where this happens reduces linearly with dwell and number of cylinders (IE a waste spark V8 will begin dwell overlap at 5000 rpm with a 3ms dwell time).
Hope this helps.