Hi,
It is very much likely that Relevanssi overrides the final results there.
Sending to a custom page would not change that, as the custom results URL option does not change the results URL, it is used in cases when there is a custom results page permalink structure is in effect, so the plugin can adopt that. It is however not possible to have two logically separate results pages, as it always processed by the same WordPress core handler, and the override would trigger every time.