Hey Growmie,
Based on the specs of the TS600 and the PPFD chart, I would say that you would be good at around 18 inches. Now, by adding the second light to the mix, their will also be light received from the coverage of the other light. Your plants look like they may benefit from having both lights set around 18 inches and go from there. 15โ seems closeโฆattempt to avoid heat stress, light stress, burn, etc. As new plants they are still very easily affected by the light. Good luck ๐ค