No, each crime should get the sentence appropriate for it and it alone.
And no, it wouldn't "cut down on the #### more than a little".
According to
RAINN, 31% of sexual assaults in the US are reported to the police (I'm inclined to believe the actual percentage is a lot lower, but that's the 'official' number, and it will do for this discussion). Of those, only 16% lead to an arrest. You can draw you own conclusions on what that means. Of those that do get arrested, 56% are convicted and 50% are imprisoned. Rape is notoriously difficult to prove in court. Even with strong physical evidence it often comes down to 'he said she said'. So a 56% conviction rate after arrest is pretty good. Compare that to robbery where only 13% of those arrested are convicted, or assault and battery where 16% are convicted. The police are obviously being a lot more cautious about 'getting their man' for rapists than other criminals.
Many people already don't report being raped due to fear of negative consequences. Far easier and less stressful to just put it behind you - and let the rapist get away with doing it again to other people. For example only 10% of male rape victims in the US military report it. In that environment, telling rape victims they will be given the same punishment as a rapist if they are not believed would have an incredible chilling effect. The perpetrator only has to threaten the victim until they recant (a common occurrence) and hey presto - the bitch goes to prison for reporting me! Rapists
love your idea.