Hi,

You can create two categories in Properties of document (tree and tab) and assign them subpages according to you requirements (first x to tab category, the rest to tree category).

Now insert tab menu webpart of main content and tree menu webpart to right side.
Set Path of tab menu content to /{0}/% and insert following query into WHERE condition in Properties of webpart:
DocumentId IN (SELECT DocumentId FROM CMS_DocumentCategory WHERE CategoryId = 6)
P.S. you can find CategoryID in CMS_Category table.

Similarly for tree menu webpart insert the same query (with different CategoryId) into WHERE condition.

Now subpages that belong to tree category will be displayed in tree menu subpages that belong to tab category will be displayed in tab menu.

If you do not want to work with categories, you can select TOP N items in tab menu (insert your query to where condition) and in tree menu you can use some inner select (select all document - select top n document).

To hide submenu in main menu you can set Maximum nesting level to 1 in Properties of main navigation webpart.

Best regards,
Ivana Tomanickova