As long as you comp can deal with it, you can run a second client with a single install. Just create a shortcut from where you MxO is installed. The pick at the chortcut properties from the right-button mouse contextual menu and add the -clone switch to the target line. Something like this:
"E:\The Matrix Online\launcher.exe" -clone
In order for this to work, you must start your main account and after the login screen you can start the second client.
Really you are paying for another activation key not another copy of the game as posted previously you can just copy your entire MxO folder to another location on your hard drive or another computer and run the second account from that.
Beyond the activation key your subscription pays for access to the servers.
Yes they are right,
If you are just looking for a way to launch the game and already have the subsciptions you just need to "-clone" the original client, Just make copies of the original shortcut then add "-clone" where they said. There is absloutely no need to have more than one install/client for all the launchers.
I also save my settings for each client launcher icon I have (currently 5 diff ones for mxo) so when i have to reinstall mxo i can just download mxo once then copy and paste the icons and im done. I can then log on to all my accounts without having to reset up all the info. (i have em set up to auto log in, patch, etc. No need to type username, password)
IF looking how to get another account without buying again, as mentioned above, just get a trial key or use the beta launcher for a trial key to get another account then sub it after its over. You already have the actual game installed so no need to keep paying to install game every time you want a account.