Here's the problem: Your main chain is almost surely going to be too short to cover the big/big combination. If you ever accidentally shift into that combination (or even the next couple biggest rear cogs), it can cause a TON of damage to your bike in less time than it took me to type this sentence.

If you size the chain to fit the 56 chainring it will probably be too long when using the 52 chainring. If it was my bike I'd switch chains when I switched chainrings.