Why is there a twenty minute time limit between posting questions?? What if I have two completely unrelated questions and need help on both?
I guess the 20 minute limit is to prevent spam. Spam in the sense that someone may post a series of spam question to fill the front page with a lot of spam. I guess it is only for low-rep users only, because I remember I had asked two question simultaneously some time in the past. Another reason could be that the SE system wants the user to put some time in formalizing a question so that users could ask a good, well written question; 20 minutes is less IMO, I, for one, usually take one hour minimum and mostly days or a year for a serious question.
Now what you can do is ask the first question and then formalize the other question in the next 20 minutes. If the other question is too small to write or you have already formalized it for some reason then you can think about the comments or answer that you would get on the first answer. If you don't get any response on the first answer, have the other question pre-formalized then I'd suggest you to play Counter-Strike 1.6 multiplayer in those 20 minutes :-p.
Another thing, do not make a new account to circumvent this time limit. Doing this will result in the suspension of your account -- This policy is mentioned by an employee of SE company here.