Well... My daughter was recently diagnosed with ADHD w/acute anxiety. She's not defiant, but I know defiance CAN come with ADHD. Like if I ask my daughter to go get her shoes on, 1/2 way there, she'll stop and go to the bathroom and while in there, decide she's going to put some lotion on her hands, then she comes back out and goes to the fridge and asks for a snack... I say, "When you get your shoes on, you can grab a snack." She then will start towards where her shoes are, see something on the table of hers, start looking through or playing with said item, etc... This can take half an hr or more. Just to get her to focus enough to get her shoes on. One would think she's just not listening. She heard me and then got completely distracted by something else 10 times along the way. Then the hyperactivity is there with her too so she'll be running around like a chicken with her head cutoff with all the distractions she gets involved in. I'd have to say for someone to be looking from the outside, it would look like I don't have any control over my 8yr old... I do. I just have to be patient and start getting ready or begin the routine earlier than what would be required for a child without ADHD. Her anxiety causes her to procrastinate if she's worried about a new setting or new people or even if it's just a new thing to do. Everyone that sees her, even if they know her well, ask me, "How in the heck do you deal with that?!?!?"
Talking with adults that suffer with the same have helped me to better understand what she's dealing with in her head and taught me how to better deal with her.
As far as dealing with your 2 kids on your own... I did it for quite some time. The relationship between my ex and I affected our daughter a lot too. He wasn't very supportive while I was married to him. But believe it or not, he was much more supportive in our daughter's life AFTER we divorced. If you have any questions, don't hesitate to PM me. : )
My husband (daughters step-dad) is VERY helpful and supportive. I couldn't ask for a happier household now. My daughters step-mom is wonderful to her too. I'm not particularly fond of the woman, but as long as she's good to my child... I don't care if her personality severely clashes with mine.
Good luck!