I just purchased a Axiom EP350 sub. Looking at normal sub low level connectors they have one connector for the reciever and one for the sub. My reciever (HK avr520) has one connector but the ep350 has a left and right. My question is what kind of connector should I use?

depending on the sub, either it will have you use only one of the plugs ( likely) or you will simply have to buy a splitter and use both

It will work fine if you simply plug into one of the connectors. You can also use a Y adapter to split the cable and connect to both.

Either way you'll be fine.

but beware, if you plug it into both with a y splitter, put the volume on your sub down, because the gain will be twice as high.