PJSIP supports multiple phones registered to the same AOR, but right now you will have to configure PJSIP manually, something similar to:
[transport-udp]
type=transport
protocol=udp
bind=0.0.0.0:5065
[301]
type=endpoint
context=internal
disallow=all
allow=ulaw
auth=301
aors=301
direct_media=no
rtp_symmetric=yes
force_rport=yes
rewrite_contact=yes ; necessary if endpoint does not know/register public ip:port
ice_support=yes ;This is specific to clients that support NAT traversal
[301]
type=auth
auth_type=userpass
password=aabbccdd11223344
username=301
[301]
type=aor
max_contacts= 2 ; max number of phones that can register
qualify_frequency=1 ; Interval at which to qualify an AoR (default: "0")
authenticate_qualify=no
remove_existing=yes
maximum_expiration=7200 ; Maximum time to keep an AoR (default: "7200")
minimum_expiration=6000 ; Minimum keep alive time for an AoR (default: "60")
and be sure your dialplan is also changed to dial multiple contacts instead of the last registered:
exten => _3XX,1,Dial(${PJSIP_DIAL_CONTACTS(${EXTEN})})
If you would like to see PJSIP support through the GUI, pleae help to reach our patreon goal at http://www.patreon.com/issabel