When you setup cross-save, you selected your Xbox account as the active save data. This would be why you cannot access your Steam characters on either platform, as that data is dormant. In order to change your active save, you will need to deactivate cross-save, and wait out the 90 day cooldown period for doing so. If you purchased silver on any of your accounts within the last 90 days, then you will first need to wait out the lockout timer from that purchase. Do not make further silver purchases during this time, or it will reset the timer. Once the timer has elapsed, you can then proceed with deactivating cross-save, as above. Feel free to read through the linked article below, for more information on the cross-save policies: https://help.bungie.net/hc/en-us/articles/360049498031-Destiny-2-Cross-Save-Guide