Drabbles are also close to poetry as I've written poetry, which usually is haiku/ waka or something rhymed and metre'd, where again you don't have much leeway. Put in a word here, take out another there: a word for a word, always.
With these constraints on you you then up the difficulty a good deal by stating the pairing when you have a theme of some kind. 'The theme is colours and it's Gojou and Hakkai' at once demands that you betray, and betray sucessfully, the readerly expectation that red or green will turn up in that one. A pro like incandescens can side-step the obvious as if it didn't even exist, but not everyone has that expertise.
I find the same goes for cross-overs. Name the two series involved and most of your readers can at once make a stab at guessing the theme. (Unless it's Onmyouji/ Teletubbies where the points of congruence don't exactly leap immediately to the eye.) That's the problem with this: I like the idea- I've always liked it, though it isn't worth more than a drabble- but it's so obvious there's no real point in writing it.
But I did. It's half Wild Adaptor:
'Or a kitten. Not for me,' he extemporized. 'A young friend who needs cheering up-- I thought maybe a pet...'
Polite smile. The man knew his store was being checked out: it amused him. Not guns or drugs then. Politics? No. The atmosphere was tourist-trade 'Chinese': incense, ceramics, cheap brocade. Aimed at the locals. Ahh. 'Perhaps you have more... exotic animals for sale?'
'I have all kinds of animals. I will accommodate your request.' He spoke as if the transaction was finished.
Kou blinked, startled, and met the owner's bright smile:
'I think a cat should do it.'