That's crazy you say and you may well be right. Let me explain. My bike has a Nexus 8 geared hub and there's only a cog just like on a fixie. Chain tension is adjusted by loosening both axle nuts and by pulling back on the wheel while also making sure the wheel is centered in the rear track fork. This is a bit fiddly and for convenience's sake, I would like to install chain tensioners on both sides.
I have a bike with a 2-speed internally geared kickback shift hub. Only 2 speeds instead of 8, but the principle is the same. As you may already know, the axle must be secured against rotating since the planetary gears inside these hubs exert a reverse torque around the axle (unlike a normal hub, which does not). That's why the axle has opposite sides shaved flat to fit in a lockring washer to prevent rotation. So (master of the obvious aside) you've got to work within these constraints.
I find the easiest way to set the chain tension is to position & tighten the axle one side at a time. One side tight, the other side loose, position the loose side. Then tighten the loose side, loosen the other side, position it and tighten it. Done this way the process is less fiddly and quicker. So I haven't found a need for the kind of tensioner bolts you show, which are often used on motorcycles.
Also remember the chain tension is never even all the way around; as you spin the pedals the chain always has a loose point and a tight point. This is because the chainrings/sprockets are never perfectly centered. So when adjusting, move the pedals to find the tight position of the chain, and set the tension at that position so it's not completely tight but still has just a bit of slack. The loose position will be even looser and that's OK.