If you want a more subtle option for your theme, maybe choose a colored invitation that's one of the overlapping colors (HP house and SW light saber) and then customize the language to maybe include a reference or 2.
I think the biggest problem you are going to run into is copyright. Both are very popular franchises and using their images without permission would be copyright infringement. I also thinking having two different themes on wedding invitations is just too much. Themes can quickly become tacky if overdone.
I think there are many ways to do a theme without being tacky. I volunteer with GalaxyCon, and my supervisor just had HP & SW theme at their wedding. No costumes or anything, but it was well done. Her ladies had paper bouquets with HP house colors and the guys all had lightsaber tie pins. There were other nods in their reception decorations.
More of the problem with copyrights is finding someone who can do it. If your were to DIY it and not make any money off of it, then it usually slides under the radar more easily. Your other thing is to use phrases and fonts rather than the actual image. I just saw an invitation that start with A New Saga Begins in a SW font. You could do this in your HP house color in order to tie the two together. Or simply choose to represent one in one thing and the other in something else. Like an HP Save the Date and SW Invitation.
I love it!!! I didn't like the stuff on Etsy or elsewhere with fantasy stuff either, but i liked sections from each one and blended bits on my own to get the look i wanted. How good are you with designing on your own? and printing on your own? I used Corel Paint Shop Pro (the latest version) free 30 day trial to do mine. It's an easy enough program to work, to get the background you want, and then i saved it as a jpg file and uploaded it to microsoft publisher to put text boxes of the artwork. (i downloaded free fronts from Lord of the Rings, Starwars, and Skyrim and used them in Publisher) I am trying to blend a "fanstasy" theme mix of starwars, lord of the rings, and skyrim we both love all 3 ... so my bouquet is a lighstaber hilt with flowers coming out of it, and the men's flowers on their jackets will have mini lightsabers as well.
and these are my invites:
the envelope is rough draft pictured, but it will be printed on the back of the envelope (not taped) Lord of the Rings font, with the White Tree of Gondor Seal
The Gate of Moria printed out on Vellum paper (debating a wider blue ribbon on none at all) sealed with the Tree of Gondor again.
When you open the invite inside has the Evenstar pendant in the background with more Lord of the Rings fonts (invite is 5x7)
RSVP card (front on top and it's back below it) Skyrim letter background with Skyrim font and on back of card Skyrim quest marker and font. Blends nice with LotR and has a Rohan feel to it...
more themed stuff will be incorporated in reception decor and ceremony signs, but for now, this is what I have... I plan on using my own HP printer and just print everything out on cardstock and vellum from Michaels craft store and cut it out and all.