Put this CAPX together real quick. It's just a simple project with 4 layouts (3 regular layouts and an Inventory layout). The 3 regular layouts share a single event sheet - note: you don't NEED to do this, as they can still have their own sheets and work. Inventory layout has its own inventory sheet.
"On Start of Layout", as suggested by Equinox80, the layout name is stored in a global variable called Last_Layout. I've made it a text variable, and one easy way to do it would be to simply use the system expression "LayoutName" to automatically set the variable to the name of the current layout.
In the Inventory Sheet, when the Back button is pressed, I simply use "Go to layout (by name)" and type in the "Last_Layout" variable.
Remembering Last Layout CAPX
Hope this is what you're looking for!