There's only one s in the name thus it makes no sense as you pronounce it. Also probably everyone else will read it /jupi-star/.
So Jupystar is pronounced like Jupistar.
Jupis, like lupis.
People try to be too clever with these portmanteaus, and end up with either something unfortunate or unpronouncable.
That's what I said. But differs to what you said before. /Jupi/ has only one stress; your previously mentioned pronunciation requires two, also two s rather one that exists in the word.