From Shillong double decker bridge is 2 hours away. You can reach Tryna via cab. Then do an half an drive to the bridge near premises, thereby trek for some couple of minutes to reach the exact spot. My advise would be stay in Tyrna overnight and plan visiting next day. Else start early morning from Shillong, and plan return same day.
The base station for the Nongriat trek is Cherrapunji, which is about a two-hour drive (54km) from Shillong. Ideally stay there overnight and then next morning drive to the roadhead hamlet of Tyrna which is 30min (12km) from where the descent starts.