Passing an array through GET while using url_to_assoc

[eluser]jhyland[/eluser]
Currently, on the website I have now (which will be replaced with the new CI site), there are multi selects and check boxes. So if someone selects two check boxes, or two options in a multi select, the URL looks like this..

http://website.com/?checkbox[]=var1&chec...ti;[]=var2

(Ignore the ';', the forum keeps putting those in there.)

That will get processed as this:

array(
checkbox => array('var1','var2'),
multi => array('var1','var2')
);

I want to use the $this->uri->uri_to_assoc option in CI, but that only does a single level associative array.

How can I accomplish the same task, using the url_to_assoc?

Also, how do I have a CI form "action" go to that type of url? Meaning, instead of it going to ?this=that, how do I make it go to /this/that/ ?

Thanks!