I see, you're right, in this case a single O-Ring would be enough to get the job done.
Here is the process.
1- Take out the safety line from it's ring
2- Take out ONE of the flying lines from it's ring which will now become your new single safety line.
3- Get a large enough stainless steel O-Ring so it can't slide through the upper rings of your bar
4-Connect both the safety line and the single flying line to the same spare O-Ring exactly like this:
5-Pass the flying line through the top right O-Ring of your bar like so:
6-When you activate the chicken loop QR, the single safety line will slide down like so:
It's not the prettiest thing and not the most practical setup, but it will work just fine and be safe as long as you have the correct size marine grade stainless steel O-Ring and you do the same assembly as described/pictured
hope this helps
Christian