Background information. I started the quests for both the Legion and the Stormcloaks, but stopped where I'm at the point where you can take an oath for both sides.

I'm at the point where I've spoken to Arngeir and persuaded him to host a peace talk between Ulfric and Tullius. I spoke to Ulfric, worked fine. I tried to talk to Tulius, but he only gives me an option to tell him I'm not ready to join or take the oath. He doesn't have more options like Ulfric. What do I do? I really don't want to take a side.

If you're on the pc typing setstage mq302 30 into the command console (~ key) should allow you to continue.

Edit: You might need to type 40 instead of 30.

What does that do? Tullius still doesn't want to talk to me about it.

Ah I get it. Thanks for setting me on the right path.

Via http://www.uesp.net/wiki/Skyrim:Season_Unending, I used setstage mq302 20 to get past Tullius'.