Good question and not so obvious in the first place!
Answer: it depends on the products the companies are selling.
- Similar/same products: overlapping customer base does not add a lot of value since cross-selling opportunities are minimal (customer from company A has already a similar product as the customer from company B could purchase).
- Different complementing products: overlapping customer base is a plus because you get access to a significant customer base with according 'big data' and you have cross-selling and/or up-selling potential.
Hope that helps - if so, please be so kind and give it a thumbs-up with the green upvote button below!