Well if your different OSs are installed on SEPARATE drives, what's the problem here?
Presumably you can boot to the Win 7 installation and carry out any configurations, updates, etc., etc. involving that OS.
If you boot to your other drive with the Win 10 installation...ditto re that OS.
But surely you know this. So I'm probably misunderstanding your query.
In terms of backing up each system, consider using a disk-cloning program to COMPREHENSIVELY backing up each system. You provided no information re the volume of data on each drive. Depending upon that total volume of data, perhaps you could utilize a single large-capacity HDD connected as an external USB device as the recipient of the clones - perhaps multi-partitioning the HDD to accommodate the total data from each OS on a separate partition, so that a single HDD could serve as the backups for both systems. Or you might feel more comfortable with two HDDs, each serving a single OS.