I have a 70-200mm canon lens with 67mm filter threads (I am using a Canon 40D so it has a 1.6x multiplier on the numbers)
I would like to buy a 18-55mm lens as well with 58mm

I want to buy a polarizer and be able to use it with both lens. I was thinking I could buy one for the bigger lens and use step up rings to attach it to the smaller lens.

I don't know it this would create problems or not.

Please tell me if you think this would be ok and if it is not ok then which lens should I buy the polarizer for? (I don't really want to spend 200 bucks to buy two of the same thing)

Welcome to the Forum. That is the way most people to it. By one, high quality, filter for your largest lens, and use ste up/down rings to make it fit on all your other lenses. I would consider buying a 77mm filter because of lot of the higher quality lenses take this size filter. You may not have a need for it now, but you may in the future.

The only problem you may have is that you may get some vignetting on the wide angle lens. Other than that, it should work fine.

While it's true that some world class lenses use really large filters, the problem with 77mm filters is the price- one circular polarizer can cost more than a year old camera body. While it's better to get the largest you can afford (within reason) if the OP has no desire for a monster lens (OK, we all have a desire for them, but if he has no plans to buy one) with 77mm or 82mm threads then his idea to go with 67mm and use a step-down ring is a good one. On the other hand, I standardized on 72mm years ago, but just bought a camera that takes 58mm filters. I bought a couple of my favorite filters in 58mm so I don't have to carry my 72mm filter wallet all the time. I also bought a step down adapter so I don't have to duplicate some of the more esoteric filters, but can take them along if I think I'll need them.

$200 sounds like a lot of money for a even a really good polarizer. I wouldn't recommend wasting money on a cheap one, but I have a Hoya SMC which has really been good - and I only paid slightly more than half that for a 77mm size. This was a few years ago, but you get the idea.

If you plan on keeping the lens with the 67mm filter diameter, then what you plan on doing is good. I'm not sure if that's a common filter size with Canon lenses but you may want to even go a little bigger if not (72mm or 77mm may be much more common). It is a little less convenient to use step-up rings but not that big of a deal. That's what I do on my 72mm diameter lens rather than spending money on a second filter. Also, you won't be able to use a hood with it, but I generally don't do this anyway with a polarizer.

when I said 200 dollars I meant 100 for each

when I said 200 dollars I meant 100 for each

Ahh - sounds much better...