I am completely frustrated with these bach two part inventions, I can't seem to put left and right hands together

. I memorized the treble and bass clefs separate in 20 minutes (ten minutes each). I've been at it for more then 5 hrs trying to combine the two clefs (Two Part 1 btw). What am I doing wrong? Should I just memorize them simultaneously bar by bar? Any advice would be greatly appreciated...thanks