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A gravity system has one motive force, gravity. If the feed is upstairs, then warm water needs to come down, which is against physics. It is the falling column of cold water that moves the warmed water upward. A circulator overcomes the problem.
The block to the heating system is the introduction of air into a location that traps air in the path. Look for such a location and eliminate the air. Better yet, eliminate the air trap.
If the feed to these two radiators is as you describe, a system would take a long time to overcome the piping problem. The velocity of water in the feed would need to increase to induce a circulation. This can take time.