A few basic rules for solar panels. You always want least flow resistance for maximum flow. Next, always input water at the bottom, take out the top. This is so air can be purged from the tubes. The 'disc' you mention is probably a valve. I can only speak for my 4x20', forget the name, but that valve when closed lets the in and out be on the same end. With that valve open, water can flow in one header, down the tubes to the header at the other end and out the top of that header. Lowest flow resistance configuration. I would run this way and parallel both panels. Maximum flow means a lower TEMPERATURE rise and the panels should be cool, but it also means maximum heat transfer to the water which is the important parameter.
Al
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