Going from one dog to two

Hi! My fiancé and I have one dog right now that is 1 year old. We have been talking a lot lately about getting a second dog. The opportunity presented itself to us tonight, and we could get the second dog within a week.

I am curious what the transition of going from one dog to two is really like. Is it truly double the cost? What are the challenges, pros/cons, tips and tricks, etc.

Thanks!