One way would be to estimate and draw the same angle going towards the left until it reaches the horizon line and that would be your 2nd vanishing point. If you want to do it completely accurately you'd have to draw out a plan or get a compass and measure the right receding angle and draw it out on the left side. Once you found your 2nd vanshing point you just connect all the angles receding to the left to the left VP and all the angles receding to the right to the right VP.
Although it's simple to use a plan you'd need a triangle or a compass to do it accurately, and usually it isn't needed for sketching and stuff like that. A good book's this one recommended by scott robertson (who also has a book comin out , http://www.designstudiopress.com/ for his ).
Check here for the perspective book, it's really good-
thanks, though this was not what i was looking for, it's the answer to my question.. i assumed there was a way of deriving (is that the right word?) the left side from the right without setting the vanishing point, but i was probably wrong. the first link doesnt work, but the second does, that book looks really promising, but the price is extreme. well, another for my books-i'm-going-to-buy-if-i-ever-get-rich-list.