So a few tests using cut up strips of plastic off an ice cream lid and grey tape to locate.
Essentially split the intake tube and then folded the plastic back towards the passenger side to force air to arc back to the passenger vent.
First go and I didn't fully segregate between the flows.
Though this caused some flow transference it wasn't terribly significant and the drivers side was still dominant.
So fully sealed between the flows and got fairly balanced airflow.
Have started making a fibreglass splitter off some metal I bent to suit the set up, as shown.
Will see if the polyester goes off in this cold though it should given I upped the catalyst amount.
Will build up the splitter once hardened to fit the duct and fully seal the flows.
I can always open up some holes to help favour the drivers side airflow if that becomes my need.
Thanks for the suggestions guys.
Cheers