srec_cat.exe from:
https://sourceforge.net/projects/srecord/files/srecord-win32

Keil: usage notes for srec_cat.exe - "MERGING TWO APPLICATIONS INTO ONE INTEL HEX FILE":
http://www.keil.com/support/docs/2666.htm

Keil: create a µVision project for downloading a HEX file to Flash - "Flash Programming :: HEX Files"
http://www.keil.com/support/man/docs/uv4/uv4_fl_hexdownload.htm