Swallows fighting over nest

Hello,

Can anyone tell me if it's ok to site a swallow nesting box within a few feet of an existing  nest? I'm asking because we have a small brick shed that swallows have nested in successfully until last year, when they seemed to spend the whole summer fighting over the nest, which even resulted in the death of one bird. There were no chicks at all. I'd like to try putting up a nest box near the existing one but I don't know if they'll co-exist in a relatively small space.

Thank you,

Susan