Respectfully, you will not be backing up anybody on a DG hunt. That is the PH's job and if things go tits up, again respectfully you have not the experience and would be more of a danger to the participants and yourself than an aid.
Have a look at some of the "youTube" films of when things go wrong. One was posted on this forum. The client nearly spines the PH. With two clients, a pissed of buff or ele and the PH it will be like trying to shoot a rat in a phone booth, somebody is going to get hurt.

Now if it is your own buff then the 450/400 #1 will get the job done in great style with correct bullet placment. You will have to accept however if it is not a great shot and the buff is heading south at speed, then the PH will proberbly have to fire an insurance shot, if it is heading towards you he will kill it. With a BA or DR then you will be able to do that yourself.

Regards