I had installed two additional scissor jacks in the middle of my Jayco (pardon), trailer.
My experience proved to be almost useless at improving stability.
I had used self tapping bolts to install them and it was very difficult to exert upward pressure to penetrate the frame.
I find that chocking the tires with well designed chocks goes a long way towards eliminating that annoying rocking motion.
Chocks should have
exactly the correct tire radius
when they are sitting on the ground. allowing for no rocking.
Also, as someone else mentioned a little while ago, pulling your rig forward slightly against one chock while shoving in the opposing one helps a lot.