User blog comment:Kaloneous/Mass Effect 3 Ending DLC Denied and PAX/@comment-1841436-20120320221509/@comment-118213-20120321050714

Read on... "However, they have fixed one-to-one connections: a primary relay connects to one other primary relay, and nowhere else."

"Secondary relays can only propel ships a few hundred light years, however they are omnidirectional: a secondary relay can send a ship to any other relay within its limited range."

The primary relay is a simple two way relay, it goes their and back. Thus requiring TWO. A second relay, can send a ship to any other secondary relay, thus again - requiring a SECOND relay.