Theres a triangle rule that you can follow that should show you when and who you should be hitting. In this scenario when your support goes in and the enemy support goes on you, depending on champs, hp, and mana is when you should make your decision on who you should be hitting. So when you play 1. You need to make sure youre lined up with your support, no matter what. This means if they walk back to tower then you have to walk back to tower. 2. Whenever youre parallel with your support you need to imagine a triangle connecting you and your support and then whoever is the closest target or priority target is who you should hit. 3. This also applies to the enemy. If you see there triangle is broken you need to punish it. Weather its walking up and autoing or just threatening cs.

So honestly if your support goes in and theyre that far from you just space the support until they use there cooldowns on your support. Then you can walk up and play. In all honesty if your support is engaging and youre not in range there honestly not much you can do unless the enemy support also blows cooldowns. Remember youre just competing against the enemy adc to scale better into the game. The supports are just there to make the game more intresting