ok just went and looked to make sure and it is as you would expect
2x twin filiment bulbs 21w/5w and
pins are offset which is what makes them go in only one way which means they are hooked to the correct circui
tail lights both bulbs iluminated
brake lights both illuminated
see how the pins are offset
switch the bulbs over first and if it still lights the same then it is a problem with the connections if the problem switches sides then its actually the bulbs
you could just check the bulbs with a multimeter if you have one but if you did then you prob would have already
you might find it easier to take the rear section off so you can clean the contacts cause it is a bit tight in there and its only a few screws and a couple of bolts holding it on give the conections a good clean and maybe a rub with a bit of wet and dry
why have two bulbs you may ask - its actually a safety feature because if you do blow a bulb then you have a backup untill you notice the failure - had to ride 20 miles at night with a lad who had lost his single tail bulb once, didnt know the lad but wanted to keep him protected from upcoming cars till we got him home safely