That's basically what I was trying to tell him in post #5. The 4 mm vents are probably adequate for each saddle tank. Once they are teed together the single vent needs to be capable of handling the flow of the two saddles. Get a 4-4-6 mm tee and run a 6mm line to a brass 6 mm tube exiting the fuse.

You can easily see whether or not this is the problem by disconnecting both vents from the tee and fill the tanks (not too full) to see if they over-pressure or not.