There is no "automated" way of doing this. But some considerations are at: http://help.filemaker.com/app/answers/detail/a_id/10687/~/filemaker-conversion-path-for-all-versions
To combine two FM files into one solution, you have to look at scripting, layouts, and schema. Each of them rely on the other. You can copy them from one file to another if you have FileMaker Pro Advanced, which I hope you do. Well, you can easily copy scripts and schema, but layouts really have to be recreated. But that aside you have to think things through. To create a layout, you have to first have schema (tables/fields). So you create schema first and then create layouts. Then you have to rethink any schema that makes reference to layouts realizing those layout IDs are now different in the new file and you will have to update them in the schema to work correctly. Note that as your are creating the layouts, they have objects (buttons) that reference scripts and you haven't created those yet. So when I first create layouts, I leave them blank and don't fill them in until after I create the scripts. That way if I cut and paste layout objects from the previous file, they will have correct references to the scripts in the new solution. If you paste scripts in before you create layouts, then any script references to layouts will be incorrect.
So remember, copy schema, then create blank layouts using the exact same name as the previous file, then scripts, then layout contents. Otherwise you have to go through reconnecting a lot of things together. This sequence will save you a lot of work down the road. Keep in mind you will have to rectify any issues where the two files have tables, scripts or layouts with the same name since that won't work in the same solution.
Thanks for such a terrific reply. I really appreciate all the detail and helpful hints. You obviously put a lot of time into your reply which is very kind of you.
I'll let you know how I come out.
Great to hear. FYI, I figured this out years ago when I was doing a lot of conversions from when FM had single tables in each solution. It really was learned more from the University of Hard Knocks schooling than being smart - haha. But that is what the forums are for us to shared learned experiences either the hard way or through brains.
Don't forget to first also recreate your Value lists and your custom functions (on which your fields, your layouts and your scripts might rely