The advantage to the split switch is that if one of them is failing, but hasn't failed yet, then it might be detected as either being more difficult to move than the other or move more easily.
With the single toggle driving both at the same time there isn't a ready comparison. Even noting a difference, it might be laid to just being a different aircraft.
Often changes are gradual and won't be noticed until there is a really big jump. Example - I took out about 2 q-tips volume of dust from around a mouse wheel yesterday. I had not noticed until the wheel was difficult to turn that it had been getting more difficult to move.
Having them both provides a possible basis for comparison - like having two watches, as long as they agree one can be sure, but if they diverge, it's an indication something needs to be checked.
My vote for a single toggle would be that it is far easier to put a water/coffee/soda intrusion seal on it.
I look at this as a plateau of interface design. They both work, they have both been reliable enough to not attract attention to them. And moving from one design to the other doesn't seem to offer a substantial advantage.