Combined braking is rare in the motorcycle world, very few bikes have this. In theory it sounds great, but when riding a bike sometimes you only want to use the front or the back brake (for instance when trail braking) so this may not be ideal. I'm not saying one is better than the other, but each system has pro's an con's.
Linked or combined ABS braking systems were for a while the more common ABS systems as they're a much easier system to implement. Instead of regulating a bunch of valves simultaneously, it only has to work one valve and can use a much easier algorithm using a speed differential between both wheels to determine the need for ABS.
Yep, you're right but in my opinion combined braking is more "stright" at the moment one need it and the technic decided how balance it. But again it's just my opinion.
There isn't any real need to do any extra work on an un-linked system for ABS work. Also, those linked systems are only partially linked and not fully linked like the brakes on a scooter. Hitting the front goes full power to the front and a reduced braking bias towards the rear. When the rear pedal is used, it's just the rear brake being engaged and not both on most linked systems. I prefer having total control and input on my brakes as there are a very small amount of times where using the wrong brake may lead to some bad times.
OP is describing linked braking system.
ABS is where the rotation of the tires is monitored, and if one locks while the other spins, the offending brake is pulsed to allow some rotation. Not sure what happens if both tires lock simultaneously. The ECU must take last known speed into consideration.
The ECU itself I don't believe monitors the ABS system. The ABS hydraulic control unit has it's own logic unit to handle braking and sensory input. The computer is complex enough to individually monitor and engage the front and rear ABS independently as the hydraulic unit has two chambers to isolate the front and rear systems. Otherwise it'd be stupid to have two reservoirs as they would equalize and mix within the ABS hydraulic unit. The older systems used a very rudimentary monitoring system using both wheels so if you locked both wheels, the logic unit would stop modulating and default open since it believes you're completely stopped. I think the newer units like the one in our bike uses a differential speed indication based on time rather than using two sets of inputs. We've had people on the forums who've engaged ABS with BOTH wheels and came out fine and felt the ABS working in both lever and pedal. It would be a relatively simple computation to do where when the signal goes from a certain charge level from the speed sensor to zero faster than is possible to reliably stop, the wheel probably locked and should engage the ABS. It just has to monitor how quickly the wheel slows down to a stop and can just engage from there. If it every goes from a progressive to sudden stoppage it knows that the wheel locked. Once you are below a certain speed, the charge from the speed sensors isn't above a set level and the unit knows that you aren't at a needed speed to be engaged. It's still monitoring the speed sensors and once the speed sensors generate enough of a charge it tells the ABS unit to turn on.
Not 100% sure on the exact details of it all since I haven't broken it down and looked at the programing, but it's a similar system that the newer cars are using now.