The way I have seen this done in mergers I have been involved in is quite gradual. First, do some quick "day one" changes, where you change branding, and make t

The way I have seen this done in mergers I have been involved in is quite gradual. First, do some quick "day one" changes, where you change branding, and make the systems interoperable to some degree in terms of logins, file formats, terminology, import/export to each other, etc. Then, gradually make changes to each system over time towards a common direction, and either they'll end up similar enough that you can pull the plug on one and nobody will notice, or they'll become two different offerings in the same family. Think of Windows 96/98/ME + NT 4, XP, etc - make them look and feel similar, different implementations that co-exist for a while. Refrain from trying to make the long term decisions and planning on day one, and just see what happens with the market, personnel, etc.