What to do if I'm prescribed to 20mg Adderall (dextroamphetamine and racemic amphetamine) XR once daily. Is it ok for me to take another about 6-8 hours later?

No. This is likely to last too long and affect sleep. I would sometimes just increase the am dose and it might last longer, or add non xr in the afternoon or take the second about 4 hours later. Another solution is to augment with strattera (atomoxetine). Discuss with your prescriber.