how to create links to filter posts with two taxonomies

Hello,
Can you tell me if there’s a way to do this :
I have 2 taxonomies (category and infotype), there are 4 terms in each (cat1, cat2, cat3, cat4 and infotype1, infotype2, infotype3, infotype4).
All posts have one category and one infotype.
I’ve created a category.php page that gives me all posts (whatever the infotype) of 1 category. I’d like to add in that page, a left navigation with the 4 infotype terms and pointing each to a page listing all post of that category AND that infotype.
I suppose it’s called a filter but I’m totally lost on how to do it.

At first I thought creating custom post type would be the best way, but I’ve been stuck at the same level. Please help.
Chris