Bridge might get perceived to be rolling and heaving as they step on it, too. Just to emphasize its perceived narrowness and height.
I wonder: will the target's perception projection power (PPP) get diluted by having to influence two instead of one? Will PPP get weaker as Tirin the Younger and Billy split up after crossing?
@Mysto: she wasn't in the mirror last time, remember, Tirin was able to locate Lil Tir and Billy via a probe during the snake round because the locations are out in the open, the mirror is a shortcut portal to each location