Odd behaviour when adding two smart dimmers in one box
I'd junk those smart-dimmers and get ones that are less...cheesy
The first problem you have to address is that you have Cheesy smart dimmers that never went through the gauntlet of testing required by CSA/UL/... and thus can't be reasonably guaranteed to not do untoward things to you, like try to light your house on fire.  Since your wiring configuration looks utterly conventional here, I'd try with a better (i.e. not so Cheesy) set of smart dimmers and see if you're still getting the misbehavior described.