I had noticed though this game where I had imbued hero and was moving both my sovereign and that hero around, that sometimes spells didn't seem to work when cast from the secondary caster.

Well it seems some of them did work, just didn't show up straight away on the screen. I have some double summoned units hear. For easy of spotting, I have a round of saves and data prefixed with 1- (from when I noticed it) and 2- from when I canceled all spells to confirm this and isolate the unlinked summoned creatures.

I shall try to replicate this when I get a chance.