Are you intending to keep the Alto as a separate playback zone? Are you using any optical discs as well, or only purchasing movies from the movie store?
Whenever you have a player that is part of a system, the movie store defaults to downloading whatever qualities you need in order for each zone to be able to play the movie at the highest quality it supports. So for example, if you had your Alto and a Strato with a 4K HDR display, and you purchased a movie like Spider-Man: Far From Home, which is available in 4K HDR, 4K UHD, and HD, your system would by default download 4K HDR (for your Strato) and HD (for your Alto).
Unfortunately, there is not currently a way to tell the system that a player that is part of your system is not actually being used for playback. This could happen if you kept the Alto purely to use its storage, for example.
If you were not intending to keep the Alto as a separate zone, and you did have need for optical disc playback, then one option would be to Co-Star pair the two players together. This is supported to allow Strato+Alto systems to handle optical discs within a single user interface (typically with a DV-700 as well, but that's not required). In this scenario, the download logic recognizes that you only have a single playback zone because the players are paired, and the download-steering logic will produce exactly the results you said you want.