In reading about Sipstation, it sounds like they only allow/limit one call per trunk on their SIP service. It also appears as though you can pay additional for their cap busting service which will allow more calls in on a single trunk. I.E. Put one on hold and answer another. It sounds as if they have it setup like good ol' POTS. I could be wrong, but that is how I read it.
You probably need two trunks for two simultaneous calls with SipStation.
Check out FlowRoute or one of the other sip providers. You can have any number of in/out calls on a trunk up to your network/Internet bandwidth limit. (usually capped a lot lower going up, or out of your network). You can a free trial there too.
With other providers, you may have more than one trunk. The second is for redundancy should something happen to the first.
Good luck!