I am a TrainingPeaks coach. In my opinion, if you are planning to just run a TrainerRoad training plan, and aren’t being coached by anyone, I personally don’t see the need for both. TP does offer more analytics, but they’re of limited value, particularly if you are following an already well-structured plan.
TR offers enough analytics for the vast majority of individual athletes. Much of the information people glean from other sources is of limited practicality. Lots of cyclists and especially triathletes are data geeks (me included), but frankly a lot of the extra data just isn’t all that actionable in any meaningful way for all but the pointiest of pointy end athletes.
That said, TrainingPeaks is a really good coaching and planning platform if you’re self coached or seeking a coach. I’ve been on it for a decade and a half, and even though I’m personally training via TR right now, I still use TP to log my personal workouts as well, and occasionally I look at the analytics in addition to TR.