Hi, I found an example of on of my own shots ( I do a lot of holiday parties). One of the things that I watch the most is the
inter-head gap. Or just tell the lady to look at his nose while I tell a joke (that's how this was done). In some cases you may need to step to one side or the other (to the right in the example below)
All valid suggestions avout DOF aside, to specifically answer your original question, use the subject's lower eyelash which is closer to the camera. That is, you can only focus on a single point while shooting.
But watch that inter-head gap (the space between the two heads)...
I'll look for more examples for you. Oh, and let me know if you have questions; NO ONE was born knowing how to do this...