Hi @Frederick,
Great questions! There have been a couple of feature requests about validation rules allowing other field values to be used:
These requests have been shared with our product team for consideration.
As far as a workaround to accomplish this, I’m unaware of one that can be done with native Knack features. It’s possible that this could be accomplished with custom code or via an integration. It might be worthwhile to reach out to a Knack expert for help with this setup: Knack Experts Network
However, I’m not sure if this idea would work for your use case, but there is an option in Date/Time fields’ settings to select “Yes, add options like repeat and end-time to use with a calendar” at the bottom. This would allow your app users to utilize one Date/Time field to select start and end dates. See the below gif for more clarity where I highlight how this option works:
(Apologies for the poor video skills 
)