The simple (and, I believe, proper) solution is just to use a single footnote for both citations.
But there are occasions where double-footnoting is called for, so here are my two thoughts:
You can do this with inline footnotes – putting them one after another (getting the superscript comma is ini any case up to you). I gather you are using inspector footnotes, which need something to “grab onto”.
Why not let the first footnote grab the end of the sentence and have the second footnote grab the superscripted comma you’re going to need between the footnote numerals anyway. Might work.
Using a single footnote for both citations was the fall back position, but I thought there might be a better way of doing it.
I couldn’t get the inline footnote idea to work so it looks as though the single footnote is going to be the only way.
One way which does work, at least with Nisus Writer, is to link each footnote separately to nearby positions in the text and then following compile, do a cut/paste to put the markers in the ‘correct’ (single) position, separated with a superscript comma. Can’t see any reason why this wouldn’t work for those using Word.
Tedious if there are several of them, but it is workable.
[EDIT] I’ve looked thorough my copies of the MHRA Style Guide and Hart’s Rules, and cannot find any reference to how to treat multiple citations at a single postition. There may be something in the Chicago Style guide but I don’t have a copy to hand.
In the Project → Project Settings pane, go to the Formatting tab and check the “Use footnote marker” box. That will allow you to create footnotes without anchoring them to a text selection. So anchor the first note to the end of the sentence, and then put the next one after the period.
Depending on your style requirements, you may then need to move the period at the same time that you add superscript commas, but at least the citations themselves will be in the right place.