Sorry mate, the NY overlay works because the Y fills the negative space in the N. When placed on top of one another, an F and R have too much shared space to have a legible overlay. Look at an R closely enough, and you can see that it is already an F with an extra leg and closed arms. Same goes with lower case f & r, too much shared space.
I know i was trying to come up with something yesterday while i was on my 12 hour train ride. Though you might have better luck.