@yauman: No, it is not tough at all - the matter of fact, you yourself resolved the dilemma - 1) you do need a second camera body as a backup, 2) it would be nice (for you, just like for anyone else) to use the same type of battery and a grip, 3) you obviously do not need a full frame for shootings "events", 4) the difference of 600$ will allow for getting two decent DX lenses...
The plain logic or a common sense leads to the conclusion: buy yet another D7100, and be completely secure when shooting events...:semi-twins:
Having two similar cameras is a great idea but if you are shooting an event, i.e. group pictures, using an ultra wide angle DX lens will distort their faces. Meaning ladies who are positioned on the edge of the camera will appear wider, people in the middle will be elongated. It just doesn't look normal to me and I hate using anything wider than 24mm but that is just me. This is ok with a FX camera and is not an issue.