Dialing two external numbers using Originate command

Is it possible to have the Asterisk PBX dial two external pstn-extensions over a SIP-trunk using the Originate command as described here:

http://www.voip-info.org/wiki/view/Asterisk+Manager+API+Action+Originate

?

If yes: How do you do that? I always get the “channel not available”-error.