If the ultimate is L1+R1, it's very easy to do on a XIM Apex.
Ensure Expert mode is enabled in XIM Manager > Global Settings.
Edit your gaming config, swipe from right to left to show the HIP page, then scroll down to the bottom of Actions.
Click the ellipsis (...) and you'll see the word Primary.
Click the arrow to the right and you'll reveal the Secondary bindings page.
Assign both L1 and R1 to the same key or mouse button in Secondary, e.g V.
Pressing V will now activate L1+R1 for the ultimate.
I had a hard time with this, I was trying to bind both to mouse forward and it wouldn't work in game. I don't remember where I read but I remember reading something saying that some actions in Apex Legends are based off ButtonRelease rather than ButtonPress, so it may have been a timing thing related to that. Either way, the issue was instead of activating the ultimate, it only activated the tactical ability, got me killed a few times lol!
I'll give it a shot again though, something could have changed, or I may have messed something up. I'll report back!
- Side note: if this does work now, or it was my config that was messed up, you still have to keep in mind that if your ultimate is not ready and you press your ultimate button, you will use your tactical ability