Single Targa vs 2 PNG

So i can’t seem to get PNGs to carry their alpha into UE4 but can with Tga. after some research it seems like PNGs dont carry a seperate alpha channel.

So which would be more efficient after packaging? a single Targa with the alpha channel included (16Mb) or a PNG for base colour and another for an alpha mask (1.8Mb each)? Is there a differance at all or will UE4 package it the same regardless?

My hard drive space isnt an issue (This project has its own 1Tb drive) but keeping it lower for the player is obviously preferable.