Preforming Worse in a Duo Que?

I'm a support main and my brother is an ADC main. We do exceptionally well when either of us play solo, but when we duo together, we just preform worse than we normally should :/

This really bums me out because my brother got me into support. I always felt bad how shitty most of the supports he had were. So i gave sup a try, and I really like it! We even have done well together in the past.

So why are we so bad together and how could we improve it?