Over them and anchor the cover(s) with BIG rocks.
Beyond that Mrs.SR claims that I could kill an artificial plant.
20 degrees? Yowzah!
I lived there (Metro Center area) for 7 years and we never did anything special for our trees...we also never really had weather that was that cold.
I believe the earlier poster is right concerning the wind keeping your trees frost-free. I’d leave stuff be and ride out the cold snap.
We had snow here in Sierra Vista this evening. My fig tree will probably lose its last few leaves tonight.