Supporting this idea! I mean we can see a warning when we start a trade and want to add pets to our side of the new trade, so why not have a warning there as well? ^^
It would help out and prevent other important trades from auto-cancelling. I especially like the idea of getting links to the trade that has the pet/s in it, as it makes it so much easier to find them without the hassle of searching through all your open trades.
So yeah, I totally support this idea. c: