I am final stages of merging two sites which cover different aspects of the same topic. The one which currently ranks lower in search engines will be the main domain (www .xyz.co.uk) whilst the higher ranked one will become a sub-site of the other (www .xyz.co.uk/abc). My reason for doing it this way is that the URL of xyz is more important and relevant than abc. My thinking in terms of the rankings following the merger is that abc will continue to be ranked high in the short term whilst over time xyz will move up and eventually overtake abc. Does anyone have any thoughts on this?
why not just go with one url or have a new url altogether? sub-sites will have its own ranking while main site its own ranking. Good SEO will help you make any domain name ranked good. What you need to do is follow a simpler strategy. Hemanth Balaji is a good SEO specialist who can advise you on this.