We have about 1800 mm to play with. The basins I like are undermount, 500 mm long each. My preference is to have the majority of space on the outside of each basin and less between them - my husband is left handed and I am right handed - he will have the left side and I will have the right to we won't clash elbows etc. But what is the minimum space between basins before it looks odd or gets annoying?!