If you just want all the sheets in their current form, but in one
file....the easiest way is probably drag-n-drop. Have both files open,
and nothing else.
Menu: Window->Arrange->Horizontal (any will work, horizontal will be
easiest to see if there are a lot of sheets)
Then select all the sheets in one file, drag them to the other one. If
they need to be in a certain order, drag them into order. Save.
It will automatically rename any duplicate sheet names with a "(2)" at
the end.