How can I run two loops? One is for the LDR switch and the other one is for the 20x4 display.
What kind of loop are you thinking of?
The usual suspects (for loops, while loops) don't make much sense for an LDR or display, and if you talk about the loop() function that loops by itself, you can have only one of those which is all you need.
Read those two and get back to us.