I have both! But when I was diagnosed with ADHD, autism couldn’t be co-diagnosed. (My mom was convinced I was autistic too though)
The best I can describe having both is that overwhelm with ADHD and overwhelm with autism feel a bit different. ADHD overwhelm is more mental like, “There are so many things to do and I don’t know where to start.” And autistic overwhelm is physical like “The lights are too bright. Too many sounds. Too many voices. And it physically hurts.”
But they are similar in many ways. I’ve found that the overlapping symptoms feel different in key ways though.