In the YouTube video below, the Stylophone player is using the second stylus to engage the 350s reiterate function, which is sort of an extreme tremolo or strumming/percussion effect. If you use the regular stylus in your left hand and the reiterate stylus in your right hand, as the guy in the video is doing, you can play two pitches simultaneously, with the lower pitch constant and the higher pitch reiterating.
The 350s is somewhat rare but used instruments can still be picked up on eBay for around $200 to $300. It requires two of an obsolete type of 9 volt battery, but it can be modified to use other types of batteries fairly easily. It is a blast to play and has some nice features that were intended to make it appeal to serious musicians. My 350s is my favorite Stylophone to play when jamming with other musicians. I will share some photos in a future post.