Thanks for the suggestion, @tjkreidl.
I'm not sure what commands I would run with this cron/systemd job/service.
I assume I would need to utilize the XO API calls to determine the list of running backups and then kill the second if the first is still running.. the issue I see with your suggestion is that my backup log would end up with many failures when I currently only get just one, if any.
While this home-lab thing is a hobby and platform for learning, I have a feeling that your suggestion would require that I invest time into learning how to and then building a Rube Goldberg machine that would results in me becoming dependent upon it, or I could let the seemingly amenable devs work on my low-hanging suggested improvement to their relatively new feature: backup health checks. I suppose I could also look into submitting a pull-request
Regardless, these backups don't hold anything critical per se; only the feeling of satisfaction I get from maintaining moderately resilient backups (I can't afford "3-2-1", but I can afford "2") and getting that sweet notification from my xcp-ng hosted internal mail server that the backup was successful. TBH, I could lose "everything" and not really lose anything because I still have the knowledge and experience and it would give me the excuse to practice settings things up again from scratch.
Also, solutions like adding/upgrading hardware to speed up backups are not options at this point in life due to financial, electrical, and space limitations. As it stands, all of my hardware is 5-10+ years old, second-hand (probably 3rd, 4th or more in some cases--several pieces were donated to GoodWill they were so poorly valued several years ago), and I have only a single 20A 120V circuit breaker powering all lights and outlets in the upstairs of the apartment -- the joys of being an American millennial that graduated high school with little familial wealth just before the great recession that has never managed to get a degree
The neat thing is that these computers give me computational power and learning potential while heating our apartment instead of turning on a heater which only consumes money. I really need to move the computational heaters downstairs for more effective heating.. one of these days!
TL;DR - After some consideration I don't think your suggestion fits my use case, but it did provide for a good thought experiment!