In WordPress how can I display multiple custom types on the same page?

You should be able to do this with two loops as you've already mentioned without an issue, so not knowing what exactly is happening when you try to run it (any errors? nothing appearing in slider, but the posts showing? etc.) then I just have to guess at a possible answer. If this is a slider that is before the main loop, you could just use query_posts(posts_per_page=5&post_type=slider) to run your query for the slider post type, then use the standard loop output to show it [if ( have_posts() ) : while ( have_posts() ) : the_post();], and finish like you mentioned with wp_reset_query(); after the loop ends to then start the main post loop. Or you could use a combination of get_posts($args) and setup_postdata($post) to accomplish the same goal. Check the codex to see if that's what you need. I do this on one of my new WordPress websites - http://NewMediaWorkshop.tv - to run my featured video tutorial header, then output the main loop below to show the normal post feed. Hope this helps!

What kind of page is this? A static page that uses page.php? A blog post page that uses single.php? Something else? It sounds like you are customizing single.php to bring a slider in somewhere above your regular post content. If so I would think you could run the query above the main loop and leave that as it is. As an example I am working on a site now that shows related blog posts on product pages. (They use a custom post type single-products.php) I use my main loop in that page to get the product info, then have a query that pulls in related posts based on which tag was selected on the product page. It pulls in the 3 most recent posts in that tag's term and shows headline and excerpt. Would you be doing something similar but trying to pull in the content (that includes a slider) for one post? Will you use a different slider on each page? Is it just one standard slider that you need to bring in? Is the slider grabbing the content from the other post type? You might also want to consider using custom fields rather than post types. http://www.advancedcustomfields.com/ is good for that. I"m not sure if it makes sense in your case or not, but take a look, it might help. Good luck.

The easiest way would be use a WP_query, you can then use the 'post_type' => 'slider' (for example) within the array of arguments to define which custom post type should be used.

Try http://wordpress.stackexchange.com/.  This is a more technical site than the codex and http://Stackoverflow.com (the parent website) is one of the best technical help forums online.