Combine 8 bit rg channels to 16 bit grey

[su_liam]

I separated a 16-bit greyscale into 8-bit red and green channels as described here and here.

After manipulating the geometry of the resulting image in another program that can't handle 16-bit directly, I need to recombine the two 8-bit channels into a single 16-bit greyscale channel.

How do I do that?

Thank you for your attention and any help you might be able to provide!