My configuration: Dual boot: W8.1 on a SSD, W7 on a HD, all user files from both W7 and W8.1 are on the W7 HD.
The only reason for keeping W7 in the air was a NLE (video editing) which did not function well in a W8 environment.
What should be the best / least risky way of upgrading my W8.1
If my experience is anything to go by, upgrading an installation with /Users/ in a non-standard location doesn't go well. See my thread here: 7-10 Upgrade installation stuck/hung
I would remove (unplug) the hdd with W7 to do the upgrade... then put it back in after and have it as a mulitiboot system.
They are on different hdd anyway so there is no point to having then dualed.