You can join as many as you like, but may I make a reccomendation?
http://www.cpavirtual.org/VirtualATP.aspThis is a co-operative called Flying Tigers. The idea is that it will get boring flying the same routes for the same companies in the same aircraft all the time. With FT, you join a VA of your choice, i'm with Virgin Atlantic, but you can also fly other airlines routes with their aircraft if you choose. However, you do all this with the same pilots number and they are all logged together as one total. The main advantage is more choice and better promotion prospects. You can literaly fly long haul somewhere, then search for a local scheduled flight from that airport and do a couple of local hops.
Here's a list of the airlines involved in the group,
Air France AF
- KLM KL
Air Tahiti Nui TN
- Air Tahiti VT
Air Pacific FJ
Aloha AQ
- Island Air WP
Asiana OZ
Cathay Pacific CX
- Dragonair KA
- Air Hong Kong LD
Continental CO
Delta DL
Emirates EK
Hawaiian HA
Icelandair FI
Japan Airlines JL
- Japan Air System JD (classic coming soon)
- Japan Asia Airways EG
- J-Air XM
- JAL Express JC
- JALways JO
- Japan Transocean Air NU
- Ryukyu Air Commuter NU
- Hokkaido Air System HC
- Japan Air Commuter JN
- Harlequin Air JH
jetBlue B6
Lufthansa LH
- Thomas Cook (powered by Condor) DE
Malaysia Airlines MH
Northwest NW
Qantas QF
- Qantaslink
- Jetstar JQ
- Jetstar Asia 3K
- Australian Airlines AO
Singapore Airlines SQ
- Silkair MI
- Tiger Airways TR
South African SA
Sri Lankan UL
Swiss LX
Thai Airways TG
VARIG RG
Vietnam Airlines VN
Virgin Atlantic VS
Virgin Blue DJ
Virgin Express TV
Oneworld;
Aer Lingus EI
American Airlines AA
British Airways BA
Finnair AY
Iberia IB
LanChile LA
Skyteam members;
CSA Czech Airlines OK
Alitalia AZ
Aeromexico AM
Korean Air KE
Star Alliance;
Adria JP
Air Canada AC
Air New Zealand NZ
All Nippon Airways NH
Austrian Airlines Group (Austrian, Austrian Arrows, Lauda Air) OS
Blue1 KF
British Midland BD
Croatia Airlines OU
LOT Polish Airlines LO
SAS Scandinavian Airlines SK
Spanair JK
TAP TP
United Airlines UA
US Airways US
Enough choice for you? They also have there own Flying Tigers forums which are very friendly and helpfull in which all pilots from all airlines can partisipate.