Nine of my bikes are compatible with each other. If my gravel and road bikes could switch wheels, that'd be great. In practice though, it's never been an issue.

I wouldn't get two bikes that are essentially the same. Maybe get a replacement that's still a good trainer but also good for something different, like gravel or touring.

Same type of pedals is nice. For me, look for road and spd for dirt and trails. Keeps the shoe closet less crowded.