My understanding is that the reason for the spring pulling the throttle to full open is that in the event of a cable break you'd rather be at full throttle using ignition to modulate the power than at idle with no power, so it's a safety thing. Having two carbs does complicate things but getting on the ground safely is the priority. I know many certified planes have full-throttle springs installed on the their carbs for this very reason. With an experimental of course that is up to the builder.

True story: a friend of mine built an RV-8A in Hawaii. He got transferred to the mainland so he sold the plane to an airline pilot living in Hawaii. The pilot really enjoyed the plane and was doing an inter-island flight one day when all of a sudden the engine just lost power, the prop remained windmilling. Nothing he did would bring it back to life. He was on flight following so dispatching the rescue birds was not a problem. He did a normal water landing and although get got knocked around a bit he was fine. While he was treading water waiting for the chopper he watched his RV-8A sink to the bottom in over 1,000' of water. He had opted to not buy hull insurance, ouch! He was picked up by the Coast Guard in 20 minutes, but the plane could not be retrieved. Post accident conclusion was that the throttle cable had detached, the last A&P to work on remembered a bit of an issue with the rod bearing that attached to the throttle arm. Best guess is that the cable detached, the engine's suction pulled the throttle plate closed and the engined idled happily all the way to the water.

So, the springs are there for a reason. Lighter springs might be ok to substitute but I'd keep the same design to make sure that they are able to stand up to the vibration.