As said resistors often burn out, there is a modified loom to stop this, well at least on mondys there is.
If it is hot the fans do run a lot, both are used when engine cooling is needed if it is hot enough, only one normally runs with air con on.
I don't think it will be the case from what you describe but both coming on also happens when it senses MAJOR ENGINE malfunction, so a dicky sensor can cause this, and unbelievably so can the PATS module, as mine got waterlogged and did this very thing, cleaned it out dry it off all ok!!
tabetha