only way is with a group, put both of his extensions in a incoming call group and use the group number as his extension or change the group number to be his extension number, but when he calls it will display a different number.

Or like KLD said you could put his desk phone on a fwd no answer to the home phone, which is one of the nt136 you have?

“I have not failed. I've just found 10,000 ways that won't work.”