In your Library, import all your music.
I've attached images to help create the views. These are very simple but should do what you need.
Step 1. click View > Add View > Add Library View...
Step 2. In the Add Library View window, pick the type of view you want (I've picked Panes (grenre, artist, album)) and enter a title in the Name field you want for your FLAC files. I've picked "Flac". Make the Location to Audio. Press OK.
Step 3. In the tree view, right-click the newly created Flac view and pick Customize View...
Step 4. In the Customize View, click ...Set rules for file display...
Step 5a. In the Edit Search window, set the rule to: File Type is flac. Press OK.
You've now created a view that will only show flac files.
Now create a view for all the other audio file types. Repeat Steps 1-4, in step 2 name it something different like "Other" or "Lossy"
Step 5b. In the Edit Search window, set the rule to: File Type is not flac. Press OK.
You now have two library views, one that shows only flac files. and the other will show the rest.