I used to assume that I had to have the same weight in each Frontroller to make sure handling is good. Then did a group tour with a bit over a dozen other riders. Overslept one morning, many people had their tents down before I even woke up. I knew that was a short day, less than 40 miles (<60km), so I just shoved stuff into may panniers without trying to organize what I was doing since I was doing it as fast as I could.

Bike handled just fine that day with panniers not carefully packed for weight distribution.

At the end of that day, was curious about my loading. Used my luggage scale to weigh my panniers. Left front pannier, 14.5 pounds. Right front pannier, 8.9 pounds. Thus one front pannier was over 4.5 pounds (>2kg) heavier than the other.

I am sure some bikes could not handle that big a difference in front pannier weights, but in my case there was no impairment to handling. Used my Thorn Sherpa on that trip.

My gym bag is a single pannier I bought maybe a decade ago for cheap, it was a factory reject. Not Ortlieb. I often ride with that single pannier on back. Front rack can make it hard to stow my bike in some bike racks. Thus, when I am not touring I remove my front rack so the single pannier has to go on back.