Currently this is not supported and is on the feature request list. But we can do this another way, but it's not the easiest thing to set up. You can do this with config switchers. Use the guide below as a reference.
1. Open your "halo3" config, bind RB to your E key, save
2. Copy your "halo3" config, it should be named "halo3 copy" by default
3. Open "halo3" config, press LB on your controller to navigate to the config switcher section
4. Set up a switch to "halo3 copy", bind it to your R key, save and close the config
5. Open "halo3 copy", enter the config switcher section
6. Set up a switch to "halo3", bind it to the R key
7. Below the bound key you will see "pressed", change it to "released"
8. Do not save yet, navigate to your button mapping section of this config
9. Bind RB to the R key on your keyboard, save and close your config
What this does is natively binds RB to E in your config. It also creates a hold switch that changes configs instantly when you press your R key. When R is pressed it changes to "halo3 copy", when it's released it changes back to "halo3". By switching with the R key you're also pressing the R key in the second config, which is RB as well. There you have it. One xbox button, two keys. This is the only way to do it right now.
