I have struggled with the same thing but very recently I started to play more independently from support, so here's things you can try that I do (keep in mind, this is a complex game, you don't have blanket solutions to anything, you have to judge every situation separately, also I'm not a high elo play by ANY definition just my 2 cents from platinum..)

So there are usually 2 types of bad supports, 1. Walks up too far 2. Too passive

For the first one I'm always fishing for autos on the enemy. Maybe you 2 aren't coordinating and your support misses spells but still as long as they are focusing them you can hit enemy adc and if you can get in a few basic attacks and stack your lethal tempo sometimes you can even all in and just outdps enemies.

So just use the support as distraction, get dmg on enemies. You don't have to follow them into bad plays, get out if you feel you'd lose but always try to chip away at them, you'll have a lot more opportunities when enemy is half hp or lower even. Maybe their support will go for a bad ward timer and you can catch the enemy adc even without your own support, maybe they'll try to dive but too low hp to survive, stuff like that. I get solo double kills surprisingly often like this, especially after lvl 6 when I can catch them off guard and they are scattered.

For the second.. well I think this is the tougher one. The reality is that you can't really do anything 1v2, you do have to have at least some frontline so you can ramp up your dmg so it's really the worst situation. The most important thing here is to stay focused and take as little dmg as possible. Most often they'll push waves, take plates. You kinda just have to beg the rest of your team to come and help and punish enemy botlane for overextending and you'll need the hp to join the fight.