I'm pretty sure selling in two different categories is not available to all categories anymore, and that computers / electronics is one of them (at a guess, I would say it's primarily because there are significantly reduced fees for electroncs over $200 when the seller has a store, and it probably helps avoid miscategorisation etc to try and take advantage). 

Edited to add - there is nothing on eBay's help page that mentions category restrictions http://pages.ebay.com.au/help/sell/two-categories.html , but I also know you're not the first to find this issue with computers / electronics.

If I create a listing as you suggested, under the select a suitable category tab in the advanced tool (which is where I am assuming you are), it doesn't appear to give the option of a second category. However, under the suggested category tab, it does give the option of 2 categories. That said, the category can only go as far as computers>enterprise networking servers>servers and I'm not sure if the other category you want is in there.

I just inserted the word computer in the title.

Yes, seems to be the case, no additional cats in computers.

You can "trick it", (e.g. sugested category), but it won't let you choose another cat.

I have all these dresses now and no idea what to do with them :).

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@ukorim wrote:

 

I have all these dresses now and no idea what to do with them :).

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Well, you could wear them, donate them to charity, or you could sell them on eBay!!! 

Seriously though, if you've made all these 'drafts', you should be able to just delete them.

Look on the bright side, think of all the fees you will save by only listing in one category.

Don't forget if any of the dresses are above ankle length you will have to shave your legs