Try it on a scale...
Take your rig to the local truck scale loaded just like that and see what kind of weights you get on each wheel. I find that unless at least 20% of the rigs total weight is on the sidecar wheel, said sidecar wheel comes up all to easily and the tipover point is just a bit beyond that. My rigs are 450 to 500 pound BMW "airhead' flat twins with a low center of gravity hooked to 200 pound sidecars with at least 50 pounds or so of tools, battery, etc. in the sidecar. With those combos I've just barely got 20% of the total weight on the sidecar wheel with no passenger. Putting a passenger on the pillion without having a heavier passenger in the sidecar would push the sidecar wheel weight below the 20% minimum. If you don't have a scale handy, you can simply follow the old 2/3 to 1/3 bike to sidecar weight rule. Using that rule, a 450 pound bike with a 150 pound passenger needs at least at least a 200 pound sidecar. But if you start with a 900 pound bike then add 300 pounds of rider and passenger on the bike, you'll need 400 pounds of sidecar, dog, and ballast to balance the rig. At 400 pounds you'll probably be pushing the capacity of the sidecar... So my guess is the passenger will have to ride in the sidecar, with the dog in the sidecar too or riding pillion. If you've got a high center of gravity rig you need to be even more concerned with this, and you may not be able to use the pillion to carry anything heavier than the smallest breeds.