Is it legal in Ontario to tow two trailers behind a pickup truck?

Short answer, yes.

There are weight and length limits. I believe they're 23 metres and 4,600 kg.

Here's the part most people get confused on...

You cannot tow more than one trailer behind non-commercial vehicles.

Pickup trucks, regardless if they're for personal use, are considered "commercial vehicles" for highway traffic purposes. An easy way to confirm whether or not your vehicle is considered a commercial vehicle are through your plates. You'll likely have a set of black lettered plates instead of the blue. If so, you have a "commercial" vehicle.