I would hope there is always three Tuesdays in evenry month!! 
I suspect you mean five...
This isn't possible with the current module. We do plan to look at recurrences in the next major release. However, this will be with the intent to simplify (without losing the current flexibility), rarther than complicating. To be honest the current code in this area is 'complex', so we don't want to make it worse. We'll bear your need in mind though, as we work through the update.
Other than that, I can only suggest scheduling two events, one for Second Tuesday, one for Fourth.
Cheers
Roger