I've had various sized womas for about 5 years now and sometimes they are housed together, but usually not. I've only ever kept similar-sized ones together, but never had any problems. I supervise feedings, but other than that I don't sweat it. Also, I've never heard of anyone having one of their womas eat another...
I think you'll be fine to keep them together, although I agree with Jaykis and like to keep all my snakes individually for a number of reasons.
Good luck and please let us know if one eats the other so next time I can offer different advice 
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- Joe Farah
http://www.freewebs.com/joefarah