DRI3 needs to be enabled for the primary GPU. The secondary GPU doesn't matter for that; in fact, as I mentioned in comment 3, Xorg doesn't need to use the secondary GPU directly at all with DRI3. It's all handled by Mesa.
DRI3 needs to be enabled for the primary GPU. The secondary GPU doesn't matter for that; in fact, as I mentioned in comment 3, Xorg doesn't need to use the secondary GPU directly at all with DRI3. It's all handled by Mesa.