[Q] SGH-T399 How do I switch my internal and external SD cards topic

Hi, I'm new to the forums and I want to know if I can switch/combine my internal and external sd cards on my SGH-T399(T-Mobile). I also have it rooted with TWRP 2.7.0.0. Also I tried the vold.fstab method but it just staying on the T-Mobile LTE screen for about 10 minutes until I turned it off. So if it's possible please expain it in noob format. Thanks! :)