Adding RAM, only boots if one old RAM is in a slot with one new RAM

Macbook A1342 started with original 2GB of RAM, I decided to go big and try to get 16 to work (reportedly, the machine can handle it). Ordered 2 8GB sticks —specs match exactly—. If I install both 8GB sticks, the machine will not boot up and makes a single beep every 4/5 seconds. If I put one of the old 1 GB RAM sticks back in (for a total of 9GB), the machine works fine (actually, so much better than with the original 2 GB). I’ve swapped the 8GB and 1GB sticks around in both slots and tried all combinations chips and slots with 8GB and 1GB and it works in all cases( so no bad chips or slots), but refuses to boot up with 2 8GB sticks installed. I tried resetting SMC and PRAM with no effect. It seems like others have had this trouble and frequently just give up and run the mismatched RAM. 9GB is much better than 2GB, can I return the second 8 GB stick if there is no way to make the machine use both of them?