Good morning all. Are there any advantages to having a double thumb rotor setup on bass trombones? Why would someone choose this over a split lever?

The thumb rotor design came first. As someone who does like the design (I have one on my dependent setup), even I think the primary advantage is that it is more compact so it fits in smaller cases. For things that don't require much technical facility but still require the low B for example, it is really easy to actuate. But as soon as you aren't playing slow stuff or are playing a lot it becomes cumbersome quick.