You need to install 2 antennas and 2 dongles:
- One antenna + one dongle to receive 1090 MHz
- Second antenna + Second dongle to receive 978 Mhz.
Please see this guide:
Howto : Piaware SD card image 3.8.0 Quickstart Guide
EDIT:
Above guide is for version 3.8.0, but is fully applicable to current ver 3.8.1 also, except that image download link is for old ver 3.8.0. The link for current ver 3.8.1 is given below:
http://piaware.flightcdn.com/piaware-sd-card-3.8.1.img.zip