Just reporting back about my experiences.
I have tried 2WD with front 9C 2810 and rear HT 3525 on 20S3P for a couple of weeks. Initially, I had the 9C axle rotating out of the front dropouts, destroying the dropouts of my Fox TALAS fork. Cause was identified as too much torque and not enough torque arm (only one). After fixating with 2 torque arms, no issues there anymore. I used the unified throttle with dual Hall sensors to 2 Lyen sensorred 12 FET controllers, fed by a single 20S3P pack. The throttle would give first a signal to the rear and at a speed of about 5km/h would also start driving the front motor. Total bike weight was about 30kg including two motors, 2 controllers, 10kg lipo pack.
In short; I did not like it.
Hill climbing was super. Just great, and with 10% hills, I felt I could climb forever. None of the motors would even get warm. I could climb them with speeds over 45 km/h, and not even going WOT. Never tried WOT, because all hill-roads here are snaking around the hills, so 45 km/h is already quite fast.
Pulling the trailer with the kids was also super. And biking with a lot of wind was - due to the large weight - also super, with great tracking.
However, for commuting and drafting through traffic, such a heavy bike is just not fun. It might as well be a scooter. The real advantage of an E-bike for me is it's capability to go where scooters and motors can not go or are not allowed to go. And these are precisely the parts of the road where a lighter bike makes a difference. Therefore, I removed the front 9C motor and the second controller, and continued biking with "just" the rear HT 3525. Initially on 20S, now also on 30S, reaching speeds >70km/h (42mph). On my 2WD, I would feel very insecure at those speeds. 50km/h was really the max. Now, with the lighter bike, 75km/h is a nice challenge, but does not feel unsafe.
Another disadvantage I noticed was the difference in front wheel and the effect on front suspension. With each bump, the front suspension had noticable difficulties dealing with it. No change in front suspension configuration (increase/decrease air pressure, rebound, more/less sag, etc) could get rid of that. Also, the added mass in the front wheel made it much more difficult to deal with more technical parts of an off-road bike trail. I do not have a lot of experience with that, but without the front motor I can much better deal with off-road trails.
So, I reverted back to RWD, and I will probably stick with this configuration. 2WD was a good experience and learning curve for me. Time to move on.