Senior developers are not a guarantuee that the code will be good. it is likely, whenever new "senior" developers will start, they want to dump all code and write it new - this is always the same story. Instead of relying on 2 senior , developers, rely on the most popular framework for writing apps, such as YiiFramework (recommended), CakePHP, Symfony or whatever...provided you want to stick with PHP. If you start from scratch, you should also consider RubyonRails or a Java-bases framework see more (Tapestry5, Playframework, Google Webtoolkit, jRuby, etc..)

Starting an app like this without any framework as a basis is economical suicide nowadays...