If you're using TK tools, you can't have an RSVP for another event. I would also recommend having a different RSVP method as TK tools have been known to lose RSVPs and entire guest lists.
I have heard terrible things about the online TK tools.
Every time I've been invited to a rehearsal dinner, mehendi party, or other pre-wedding thing, it's been a paper insert in the wedding invitation. Either a separate RSVP card, or multiple check boxes on one RSVP card.
"I'm not a rude bitch. I'm ten rude bitches in a large coat."
Re: rehearsal dinner
TK's products are notoriously glitchy. You should find an alternative now. I think for something as formal as a wedding, paper RSVPs are appropriate.
It took 10 minutes to set up. and most people rsvp'd the same day it went out