If you control two Enduring Scalelords, putting a +1/+1 counter on one of them will cause the ability of the other one to trigger. When this ability resolves, you'll put a +1/+1 counter on the other Scalelord. This will cause the ability of the first one to trigger. This loop will repeat until you choose not to put a +1/+1 counter on one of the Enduring Scalelords.