The concern with cooking a food item containing haram ingredients alongside a halal item in the same oven lies in the potential mixing of haram substances with the halal food. While using separate knives may show diligence, there is still a possibility that the ingredients in the oven could melt and mix with other products. If elements like oil or cheese from the haram pizza melt and mix with the halal pizza, the latter would no longer be considered halal.
This matter can be resolved by obtaining detailed information from the business owner, particularly regarding the oven. If there is no chance of such mixing, the halal pizza can be consumed with peace of mind.