First talk to the second company; if they don't give you a drop-dead date for a response, give them one yourself. Then tell the first company that you will let them know by two days after you find out about the other job. This will give you at least two days to weigh your options, should you get both offers. You're doing things properly, so don't worry. They won't rescind their offer just because you want more time; they will probably, however, push back if you can't give them a date for your response.