Pairings that I use for (infrequent) weddings & events:
5D3: 17-35 f/2.8, 7D: 24-70 f/2.8
7D: 17-50 f/2.8 Tamron ( or 17-35 f/2.8 ), 5D3: 70-200 f/2.8
For sports, especially motorsports:
5D3: 70-200 f/2.8, 7D: 100-400
5D3: 100-400, 7D: 300 f/2.8 with 2X TC
5D3: 24-70 (For pit photos), 7D: 100-400
If I did a lot of weddings and events, I'd buy another 5D3. My previous pairing was a 5D and 40D. Unlike the 5D, the 5D3 makes an excellent sports camera.