If several professionals worked on the same deal, can I sue more than one for the same loss in Ontario?

Yes. If a lawyer, appraiser, engineer, or other professional each contributed to the same loss on a single transaction, you can generally name all of them as defendants in one claim. Ontario's Negligence Act allows the court to apportion fault among multiple negligent parties as percentages, reflecting how much each one's conduct actually contributed to your loss, rather than forcing you to pick just one to sue.

Importantly, defendants found liable for the same loss are generally jointly and severally liable to you, meaning you can recover the full judgment from any one of them (subject to court process), and it becomes their problem to sort out contribution among themselves afterward rather than yours. This matters practically if one defendant has limited assets or insurance - you're not stuck collecting only that defendant's apportioned share. Because these cases often involve competing arguments about who was really at fault, they tend to be more complex and document-heavy than a single-professional claim, so identifying every professional who touched the relevant part of the transaction early, and preserving their files, is worth doing before memories and records disappear.