Mixing the two in equal amounts is as good as any other solution, but I would probably just go with either 20L or 40L, the difference is not that great in color or flavor. Don't sweat it, your beer will never be the same twice anyway (well, for most of us without automated or computer controlled brew and fermentation rigs...and even then).
I personally like the effects of more/darker crystal malt, so it would be 40L (or more likely 80L) for me.
It will be delicious....and you can use the other one next time.