How hard is it to go from 1 baby to 2 relationship tester?

I have a 21 month old and I'm due with number 2 in 2 weeks! Neither was planned and hubby and I had a lot of fights when our son was first born. We are kind of hitting a rocky spot right now and I am really nervous since the new baby is coming and we are already fighting a little (nothing crazy) what's going to happen with our relationship.. I know it's going to be hard and I know we'll get through it... did anyone else have marital problems when the stress of a new baby came? And how much harder is having 2 compared to 1? Is it really a big difference? I'm getting my tubes tied so this is our last one.. we have a son and this is a girl!