NBC, more on Richardson venting

Fizz

Have a Richardson System with 2 main loops with dry return for each. The system called for a vent for each dry return but mine has common vent for each above the boiler. I notice that long loop which has 137 sf of radiation attached heats quicker and better than short loop which has 220 sf of radiation vented to same vent. My question is would adding another vent to short loop return improve the efficincy of the loop?
There are no vents on supply main, and only 1 vent serves both dry return loops.
Pictures attached. Thinking of venting dry vent by either dropping it down to wet return and capping-off to common vent, or tapping-in where it runs from last radiator on loop in 2nd pict. If I do latter, will tap-in require capping -off common? Also, other possibility is to drop-down this return and run-around perimeter of that part of cellar(about 45').