Using svn:externals you could structure your repository in such a way that the commom parts are stored just once in the source control system, so changes made can be synchronised with just a single commit and update cycle.
For example, imagine that you have ^/ProjectA
and ^/ProjectB
, each of with require ^/Common
as a sub directory.
Using svn:externals, pull ^/Common
into both projects.
The exact nature of doing this will depend on the version of svn you use, and any client you use (such as TortoiseSvn). Refer to the relevant edition of the svn book for specifics.
The ease of implementing this will depend quite a lot on how separate the common code currently is in your application; and pulling in directories as directories is much more practical than trying to pull them into an existing directory; and unfortunately wildcards for filepaths are not supported.
However, based on your description of your aim; this is the most straight-forward solution I can imagine.
Hope this helps.