At the moment I feel one us is enough, otl with rice sock between shaft and leg and penis twisted so you use the device head on the ventral side so it’s reaching the cord on the dorsal side. This seems to give a better heat and faster for me personally.
When using a fulcrum I find irl superior for this. The shaft is spread so thin across the fulcrum I feel the irl penatrates realy well.
Which US device you have?
I have a US Pro 2000 second edition and I am finding very easy to reach 40-41 degrees steady with just one transducer and no rice sock. I do use an IRL over to heat the environment but not specifically to heat the tissues.