seperate excel 2016 for 2 window

excel 2016    i open 3 files  excel  with 1 excel   and then i open new excel   open 3 files 

but when i choose switch windows  i see 6 files in 1 excel.

how to set see only 3 file in 1 excel and see 3 file in other excel

old excel can use  alt+tab  to see 2 excel that have 3 file for each other