Big/Big means using the big chainring in combination with the big cog on the cassette (cross chaining). When doing so, the chain is at an extreme angle and the RD cage will be at its maximum extension. If the chain is too short and the RD cage is fully extended and cannot spring back. Spinning the cranks in this combo will tighten up the drivetrain/chain and it will make alot more noise, worst case scenario it will lock up the drivetrain or at least make the chain hard to turn. That's how you will know the chain is too short. So it's best to check it while in the workstand beforehand. Not out on the road.

Big/big is not a combo that should ever be used for long periods. or ever if yuou can help it. Because it causes premature wear of the chain. And it is the noisiest gear combo.

But the chain still needs to be the correct length if you ever do click into that combo by accident.