Room B3 (2) "Backtrack"

Two portal ends, two receivers, but only one emitter.

One of the receivers is easy to hit; the other might require more thought.

For a particle to head downwards out of a portal, it will need to enter the portal travelling left.

The particle will be travelling left after its first transit through the portals.

Line the portal ends up so one is directly above the emitter, one is above the receiver, and both are horizontally aligned with each other.