So, I just wanted to test out some killcams and stuff. For this, I've loaded a character in Whiterun and just slaughtered everyone... well, I also started spawning single bandits and stuff to kill (cause I've read during my research that they only trigger if there's only one enemy in combat with you... not sure if that's true, though). Whatever I did, I did not get a single killcam when I was dual-wielding, not even when I've used the consolecommand "player.set killmoverandom to 100", which sets the probability of a killcam to 100%. I've done at least 200 kills or so in total, and didn't get a single killcam.

However, I kinda figured that I have around an 80% chance or so to trigger dual-wield killcams IF I only attack with my mainhand weapon for the finishing blow (I was only using dual-attacks before; both normal and power-attacks). Still, while way more frequent, even that didn't work all the time.

Of course I have googled and digged through the uesp-wiki as well when I didn't get any killcams, but didn't find any mention of why killcams might not trigger. So eventually, I've found out (as mentioned) that apparently, killcams only trigger with mainhand-only attacks... and even then not always reliably for some reason.

Do you have any insights on this topic?

I can get killcams even when fighting ten draugr in a bossroom, which can be very inconvenient...

I don't have any insight into why it's behaving infrequently when using your main hand, but since you're using PC, you could install the Violens mod. It allows you to set the killcam percentage chance (like you do through console), and it also allows you enable killcams for offhand kills. I've tried it for dual wield before, with the added change of enabling beheadings without perks, and I got plenty of killcams. It's an avenue to use if the vanilla version just won't cooperate.