For the first step maybe libsdl-dev package should be split into libsdl-extra-something (for all arch independant scripts, docs, headers) and just keep libSDLmain.a libSDL.so in the new multiarch locations /usr/lib/$ARCHITECTURE-linux-gnu/
But then you should take care of migrating every dependy to multiarch as well.
Maybe this won't be so easy after all,
For the first step maybe libsdl-dev package should be split into libsdl- extra-something (for all arch independant scripts, docs, headers) and just keep libSDLmain.a libSDL.so in the new multiarch locations /usr/lib/ $ARCHITECTURE- linux-gnu/
But then you should take care of migrating every dependy to multiarch as well.