You can get it several ways.
1. Search for the song in the iTunes store and click the Cloud symbol to download it.
2. Right click the Purchased playlist, choose Check for available downloads.
3. Manually copy it from one device to the other.
I would go into your preferences and make the secondary (and Primary) iTunes download songs automatically. Edit->Preferences, Store tab, under Automatic Downloads, check the Music box and the Always check for downloads box.