you could do that. there is a posted cisco config for using a 515 or higher (for the 3+ interfaces), and hacking a config where you have the different tunnels terminating on different interfaces as a workaround.
allegedly, this will be fixed in pix os 7.0, which is supposed to be a major revision, so I would not hold my breath for it.
depending on your situation, you could think about doing that, or configuring the remote pix to accept connections too, and telling your users to connect to different pixes as needed, or, just hope and wait for pix os 7