I think you should wear both outifts and what you like, especially if you have bought them! Trust me, I'm in the same boat and people are putting in their cents about why I did this...or why I did that.. and as annoying as it is, if you love your outfits, then don't second guess yourself about it. I'm sure your colours must be different right? Make sure you go really simple on the mehndi, so flower jewllery and light coloured makeup so it will look different from your wedding day. I know a lot of brides are wearing heavy heavy jewllery these days on their mehndis as well but just to differentiate between the two, go simple one day and heavy on the next. If you really want to wear both those outfits then I'd say go for it! :)
I was going to write a reply but then I realized SanaZaidi360 has very adequately said everything I wanted to. :P