The apostrophe comes immediately after the person or thing that's doing the "owning". If Jane is doing the "owning", it's Jane's marmalade. It would be Janes' marmalade only if there were more than one Jane (i.e. the marmalade of the Janes).
Where something is owned jointly - by both Val and Jane in this example - only the final person in the list of owners is given an apostrophe, so the correct version is "Val and Jane's Marmalade".
Introverts unite! (Separately, in your own homes.)