There is no spacing between 2 fonts. I can't even begin to imagine what you could mean by that.
"Line spacing" is called "leading" in InDesign ("Line Spacing" is generally understood as a multiplier of the 'regular' leading, as in "line spacing: double"). Searching this in the ExtendScript Toolkit Help returns a lot of items, which all have one thing in common: it seems to be a property of objects of type 'Text'.
Knowing that, there are a million ways to read and set the leading, for example, on some specific characters:
app.selection.parentStory.characters.itemByRange(5,10).leading = "12pt";
This is not very useful, as you can see when you do this with InDesign itself. Usually leading is applied to entire paragraphs:
app.selection.parentStory.paragraphs.leading = "5mm";
or even entire text frames:
app.activeDocument.textFrames.paragraphs.everyItem().leading *= 2.0;
(this last one doubles existing leading).
"Leading" is a special property, in the sense it might have two entirely different values. Either it's a regular numeric value -- a measurement in points, or given explicitly (as above in the '5mm' example) --, or it's a special constant named "Leading.AUTO". This "value" corresponds to the "Auto Leading" in the user interface; you assign it just like the other 'real' values.
app.activeDocument.stories.leading = Leading.AUTO;