Second alpha track matte

You have two options. Combine both layers you want to use as a track matte in a pre-comp then use the pre-comp as a track matte or use set matte. You must make sure that the layers that you use for set matte are not scaled or out of position. If they are then you have to pre-compose the layers to use set matte. You can also use multiple copies of set matte. You just have to adjust the compositing options. Here's everything I did to use Set matte twice. The first layer used as a matte is a black and white image generated from Keylight and I'm using luminance, the second layer used in Set Matte is a text layer.