Both Tenor and bas on the same line

I have tipped Angels Carol and in the original score there are only line for Sopran and Alt in the second line for Tenor and bas.
I want to make a sound track so that I better can learn my bas voice. But when I play both tenor and bas sound. I do not know how to put the letters S.A.- and T. B in front of the line.
Can anybody help me I attach 2 files the one I have tipped and the printed score