Tweeking Postsincategory plugin

I know that you can call or sort a post by the category. I need to call and sort my posts by 2 subcategories. eg. I have a parent category called games with two children categories called water games and beach games. I then have another parent category called age group. This has two children as well called 10-12 years and 13-16 years. I need to show the posts that fall under water games and 13-16 years. Is there a plug in or what is the code to call 1 post with muliple categories selected.

I have posted this several times and no one has responded. however my friend said that I could maybe do it with postsincategory but I need to tweek it. So if you can help me the PHP code that would help alot or if you can tell me how to tweek the plugin so that it cna read from to categories for one post that whould be great as well.