Sorry, I can’t answer your question about Komplete 10 definitively, but if it’s a proper 64bit VST2.5 or VST3 plugin it should be ok provided Komplete is installed properly and its dongle(s) are in place.
If you purchase an ‘upgrade’ it will change your existing license into one that can run Cubase 11. For the upgrade to work, you literally have to plug in the dongle, run eLisencer, put in a code, and it ‘changes’ the existing Cubase key to one that supports Cubase 11.
A Cubase Pro 11 dongle can also run Cubase Artist 11, Cubase 11 SE/LE/etc. It can also run older versions such as Cubase Pro 7, 8, 9, and 10 and their little brother versions of Cubase (Artist/LE/Etc.).
No, you won’t be able to split it up over two different dongles if you ‘upgrade’ that 8.5 key.
Yes, you CAN move the dongle from one machine to the other, and use any version of Cubase it unlocks on any ONE MACHINE AT A TIME that you like. Your upgraded dongle with work with both Cubase 11, and 8.5 installations (Keys unlock the same version and suite level [Pro/Artist/LE/Etc.] and below as far back as version 7, maybe even further but I wasn’t on board until version 7 so cannot confirm it would work for version 6 or before).
If you need both machines to run Cubase of any version simultaneously, then you might elect to buy a whole new key for Cubase 11 instead of ‘upgrading’ your 8.5 key.
Another option if you need both machines to run it at the same time, but don’t need ALL the bells and whistles of Cubase PRO on both machines, is to upgrade your Pro key to 11, and then buy a key for a smaller version of Cubase to run on the second machine. I.E. Audio tracking exclusively done on a machine running LE that is synced to a second machine running Cubase Pro which is hosting the VSTi rich tracks.