Maybe this two threads can help with that. I havent done it with elk, never hunted one, but it works on whitetails. The trick to take the tenderloins out is to stay very close to the back ribs, with the knife edge aiming towards the bone, in this way you will stay out of the peritoneum which is the layer that surrounds the guts, and with gentle dissection with your fingers you can get the tenderloins out whithout the need of using a knife.
I found one of this threads in this website and the other one on the net, there are some videos on youtube also.
Hope this helps.