Responsive and non responsive designs – two themes?

I have a client who has very specific needs for the way his website will look on smartphones so he’s comp’d the responsive side first. I can achieve that in PremiumPress Responsive Directory but what he wants on the non-responsive side isn’t compatible with features available in Responsive Directory without breaking what he wants for smartphones. Following me yet??

Yes of course it could be done with extensive CSS or programing a child theme but I don’t have robust CSS skills. That’s why I am using PremiumPress!

Has anyone ever run into this before? I think it will become more common as site owners have very specific needs for mobile/responsive in the future.

Does anyone have experience with designing a site in wordpress using multiple themes with one detecting non responsive browsers and another detecting responsive/smartphones-tables-ipads etc?

Any suggestions welcomed.