I have 2 sites, http://www.weeweeslap.com and http://www.coastercrazy.com
both use the exact same set of files and same database, all that was changed was we added a variable to see which domain it was being accesed from. Then with that we took it into config.asp and did if and then statements that if wws.com was being viewed then display this config(hard coded) else if ccc.com was being viewed, display this config (not hard coded, uses db configuration) all from the same set of files as well, only a fake ip was used to get it working on hosting account. also for the headers same thing with the variable that we created to check which domain was being used. Currently we've done also some more changes to show the flexibility of what we can do

Anyway, that's a rough idea of how to do it.
and no css was used, well only to edit the links to our liking

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