The odds of both of them relapsing are high. The odds of at least one of them relapsing is even higher. The odds that co-signing on an apartment for two addicts is going to royally screw you over are extremely high.
It's your choice. Just don't be mad at them if and when your left with the bill/legal trouble/what have you. It's kind of like the allegory of the woman who let the snake in her home and it bit her. She asked how he could do that after she'd shown him such kindness and he said:
"Shut up, woman. You knew I was a snake all along."