I think you have to look at what positions you have to backup and the different scenarios that can happen..
Assume that you keep a backup T and a backup G/C.
1) Lose a T, the backup T comes in.
2) Lose a G, the backup G comes in.
3) Lose a C, the backup G/C comes in.
Those are the easy scenarios.
The double contingencies:
1) Lose 2 G's - move a starting T to G and use both your backups..
2) Lose a G & C - .move starting T to G and play backup G/C at C.
3) Lose 2 T's - fill from FA or PS.
4) Lose a T and G - play backups
5) Lose a T & C - play backup T and backup G/C
Here's an another thought: If you keep 2 backup tackles, you are looking at the following:
1) Lose a G - Move a starting T to G, and backup T
2) Lose a C - Move a starting G to C, a starting T to G and a backup T
3) Lose a T - play the backup T
Double contingencies:
1) Lose 2 T - play backup T's
2) Lose 1 T and 1 C - Move starting T to G, and starting G to C, and play backup T
3) Lose 2 G's - Move starting T to G, and pick up a G or C from FA or PS
4) Lose a T & G - Move starting T to G and play backup
5) Lose a G & C - move starting T to G, play backup T and pick up a G or C in FA or from PS
Either a backup T & G combo or a 2 backup T combo leaves you with a hole in a double contingency situation. Which hole do you prefer?