My husband fell a lot as a child turned out to be very very long sighted. Took his mum ages to actually get it confirmed cos the doctors said the same or their 'just clumsy' nope ge literally couldn't see!!
My 2 year old falls but he scrapes his knee, he doesn't bump his head or get knocked out. Have you managed to film any of the times or write down exactly what happens? Usually my ds is just running too fast or being silly / not looking properly. Might be helpful to doctors if they can see what happens to determine whether or not they are standard toddler accidents?
That's unusual I would go to GP and strongly word your concerns. I have a 2yo who this morning took a rather spectacular tumble that would have seriously injured an adult or older child but he bounced back up with a few tears. (DH is a dr and checked him over) I took longer to recover from my horror of it happening than he did from the fall.
I have 2 adventurous kids who have taken many a spectacular tumble doing daft things but bar the odd graze or bruise they have been fine.
To have been knocked out twice by the age of 2, I would be concerned & investigating further. It may be nothing (fingers crossed) and just bad luck but I'd want anything else ruled out.