ADD is highly heritable that means there is family transmission.
Most of the cases of ADD are considered as genetic disorder, but, there is a little percent of ADD cases that are though to be acquired after conception.
Do you have information about brain injury (toxins or physical trauma) prenatally or postnatally of people who are planning to have a child together?

No. My daughter is ADD and seriously dating a guy who is ADD also. I know it is passed through family transmission and most likely if they were to marry their child would be ADD. But what I am concerned about, is there a greater risk that there may be severe learning disabilities passed on to a child because they both have this problem?

perhaps thats something you should ask a doctor. its possible that it could skip a generation... and i wouldnt really worry about this at the moment. since youre asking that question if it did cause severe problems would you be upset about it?

It is important to remember that a person with a learning disability can learn.
There are informations that more and more adults with LD and ADD are going to college or university and succeeding. They need proper accommodations and support and only that way, adults with learning disabilities, can be successful at higher education.
It will be helpful for your daughter if she plans pregnancy to know that environmental and chemical exposures during pregnancy are great risk for brain damage and learning disabilities.
These include cleaning chemicals, home pesticide use, cosmetic chemicals (such as nail polish and perfume), prescription medications, artificial food additives, chemicals in plastics, synthetic perfume and cologne ingredients, job chemicals.
I'm aware that she can't avoid everything, but she can watch out for some risk factors.
Analysis done for learning disabilities, indicated that LD is strongly familial, but the type of disability (reading/ math) is not directly inherited.
Hope you'll find the informations helpful.
Best wishes!
Marija