Alright, well I popped two fets, but I learned some things. . .
So I set the 40% 60% duty cycle. It improved a bit (the RF watts went up to 460W from 410W and the efficiency went to 80% up from 74%), so I changed it a bit more and then they popped immediately. I wasn't able to even read the amps. I have to change the duty cycle for each wave individually. Is having them not exactly the same or overlapping a huge problem? I got a new high power current limiting supply, so I'll use that in the future. I am not 100% sure that it was 40 60, but it should be pretty close. My scope doesn't have any fancy stuff. I just had to read the ns/division and do the calculation. I am surprised how little of a change beyond the 40 60 did it.
Anyway, here is the data I gathered:
The DDS exciter is making a decent looking square wave. It has 7ns of rise time regardless of frequency (from 3 to 8 MHz it stays at 7ns). And . . . the drivers have a 20ns rise time (again - regardless of frequency). The fall time on the drivers, does however, seem to get even slightly worse with increased frequency (goes up to like 25ns). So I think the drivers are the culprit. I didn't do the exact math, but I'm thinking 20ns is probably too much for 7 MHz. Is this correct?
If so, then I'm on to making a new PCB. I am going to use the drivers stretchy suggested and put them as close as possible to the fets. I gave the datasheet a quick look and the pinout looks confusing. I am not sure how to hook them up. I addition to the new drivers I am also going to have a custom crystal on the main PCB with an inverter to generate the second wave. It's either going to be the SN74AHCT14N or SN74HC132N for the inverter.