The following information will be true for version 3.1 of Sitefinity:
1. There aren't any problems to use two membership providers with a single role provider. This way you have two groups of users that share same roles, if you give a role the rights to view a page users in this role will be able to view it whatever membership provider they come from. To do that, you'll have to deny anonymous access to the page and allow "view" permission for the role.
2. If you use the Sitefinity membership provider, users will be able to log in to the admin part using only the Sitefinity login form (which uses the Sitefinity membership provider) , so users that logged in using other login controls won't be able to enter the admin part. If you don't want users to login to the admin part, maybe a better choice would be to use different membership providers.
Also, a deeper look at these security issues will be provided in the developer's manual.
About the permissions for your modules you can check this forum thread
Hope this helps.
the Telerik team