This strategy has one flaw.
Let's see the sequence of events:
- Durant and Gothitelle are on the field.
- Durant entrains Truant onto one of the opponent's Pokemon.
- Durant dies to an attack, and Gyarados comes in.
The rest goes as you have said. However, this means the other Pokemon is free to do as it pleases, as it sn't Truant bound. This can prove very difficult to control.
Often both Pokemon play an active role in doubles, so just Truanting one is usually not enough. Phazers and Hazers like Skarmory and Amoonguss can completely demolish your set ups, which could even cost you the game.
Hope I helped!