If he's talking about an addr that's been dormant and he just wants to monitor where the funds are being sent, I don't think it has anything to do with whether they're mined in the same block or not; the blockchain already handles that.

But if he's monitoring the dust transaction for other purposes and wants to include it in the next block together, it's not really a question of setting the same fee being the most effective. Instead, it needs to be a better-developed bot that quickly builds the dust transaction dynamically based on the size of the transaction being monitored. The fee depends on whether the bot assumes you have a fast parameter or if it checks every x seconds.

If it doesn't monitor in real time, setting more fees gives it a better chance of both being included equally.