The reason to operate all four valves at once is to satisfy port-time-angle-area gas flow requirements. Using only two of four valves cuts port/exhaust area in half, greatly limiting engine power. Either way, the valve train apparatus is complex, heavy, difficult to cool (must cool valves, pistons, etc. in one revolution), and expensive for a two stroke. If there were a great advantage to valved two stroke technology Mercury, OMC/BRP, Yamaha, etc. would have it in production.

Side note: Another lighter, but equally complex and still expensive approach would be a sleeve valve like those used in the Bristol radial arcraft engines of WWII. These eliminated valve springs & poppet valves entirely. Again, if there were promise it would already be on the market.

Keep thinking.........