There is no problem in putting your aircraft on someone elses AOC, it gives you an income from the aircraft whilst you remain priority for your requirements. It's easy especially if the AOC operator already has the type on its AOC so there is no need to start writing sections to manuals.
The C421 is a single pilot aircraft, so no co-pilot is required. However, if you want to run it as a 2 crew operation then that's up to you, but anyone occupying the right hand seat would not be able to log that time or use it towards the issue of the ATPL (to meet the 500 hour multi pilot time). You can change that by having a set of multi pilot SOPs written up in the ops manual, if you do that then the co-pilot can log the time and use it towards an ATPL. The ATPL cannot be issued though until a multi pilot type rating is added to your licence. You may struggle to find an AOC operator with the C421 & C421 multi pilot SOPs & who's willing to share their AOC
It's a bit of a minefield, any questions feel free to PM me.
Regards