I agree with rabbut, depends on stats. If you have any bad stats in your source water, like ammonia or nitrates, it can change the picture of what size/frequency of water changes you want to do.
In general though, having two is great because of being able to alternate maintenance (as said) and also as a backup to each other, completely separate advantages from the main one of more biomedia capacity.
The wet/dry feature of biowheels is a really clever idea, but the couple of small disadvantages I've read with respect to those are that first, if the power goes out when you are not home, the bacteria on the part of the wheel left up in the air will dry out and die more quickly than in a traditional filter. Secondly, I believe the biowheel design is one of those that tries to package the media in "frames" or some such proprietary structures (correct me if I'm wrong, have never owned one myself) which turn out to give you much less flexibility in media choice and various swapping activities. In contrast, some of the plainer aquaclears are more simple, flexible media boxes and of course cannisters are by far the most flexible of all usually.
~~waterdrop~~