auto start windows install when USB has multiple partitions

That is not a default boot.wim.
You can do a lot of things on a customized boot.wim

if you are editing boot.wim, the ideal place is editing startnet.cmd file... this is the preferred option. i haven't thought about splitting install.wim file and have not tried it. need to check on it

I don't know how to split install.wim either. I just boot into WinPE on the first, FAT32 partition and then run setup.exe in the mounted ISO file on the second, NTFS partition.

Editing "startnet.cmd" is too difficult for an idiot like me. I just copied "$_Matthew.cmd" into the "Startup" folder.

sandeepshekar said:

if you are editing boot.wim, the ideal place is editing startnet.cmd file... this is the preferred option. i haven't thought about splitting install.wim file and have not tried it. need to check on it

Attached is a small CMD file to split install.wim
Move Install.wim from \sorces folder to any folder, copy Split.cmd to same folder and then run the Split.cmd
When done, Move the install.swm files to the \sorces folder.