there was 4 trays of spearmen engaged with my carrion slurm, i had arbus hanging back waiting for his moment, the next turn arbus charged in and engaged the unoccupied front edge of the spearmen, would i treat both units as having addition threat?, i did and i slaughtered them, we couldnt find anything in the rules forbidding this, but i know the interweb will school me

No, threat is per unit. So Ardus would still be threat 2,and Slurms would still be threat 1.