On Page 54, under "Universal Effects", it states that all actions automatically gain the line

Two Banes: Suffer 1 fatigue if the check was physical, suffer 1 stress if the check was mental

On the next page, under "Banes", it states

One Bane: Suffer fatigue or stress

So which is it..? Is there a difference between these two?

It's 2 banes can incur a loss of one stress or fatigue as appropriate (and two boons a recovery of one as appropriate).

I do not interpret the lists on page 55 of the Player Guide to mean "with one" bane or boon, the bane and boon symbols are just being used like bullet points and not meant literally "for one". Also that list is all "suggested freeform stuff, ideas to use creating custom situations and location cards" etc. and not core rule.

I take most if not all of the "suggested" effects for boons and banes, to be just that, suggested results. I don't even always inforce the universal effect of "suffer 1 stress/fatigue" with 2 banes as I find it doesn't alwasy fit the story.