What happens when you have a 4 ohm and a 2 ohm sub hooked up to an amplifier?

Yes, you can wire a 6 ohm sub with a 4 ohm sub to the same amplifier, but you need to verify that the amplifier can handle the resulting impedance of 2.4 ohms - many cannot, and could be destroyed. Also, realize that the power in the 4 ohm sub will be greater than the power in the 6 ohm sub.