Merging 2 win 8.1 x64 wim into 1

Hey guys,

Since i wanted to use the rt5 of windows 8.1 for my updating,

And unlike the dvd they provide it only contains pro or core,

So i downloaded both core + pro seperate converted esd to wim and tried to aio them.

But it keeps telling me its unable to locate a sources folder in one of the wim files ?

Any clue how i can merge them then ?

And if this is really a bug in wintoolkit or simply 8.1 is not supported this far ahead ?

Thanks in advance,