The advice above is correct, but really the best workaround is not to have any disbands.

This case is covered in the [url=http://webdiplomacy.net/faq.php#faq_2]Game Rules FAQ[/url] ("What happens if I order to build/destroy two units in the same territory?" and "If someone has to destroy a unit, but doesn't enter destroy orders, what unit is destroyed?"). The engine will accept the first disband, and then pick another unit for you.

I'm pretty sure you can destroy the same unit twice if you have multiple units you have to destroy, I'm not sure if this works but it may be a problem. Once the turn ends I'll see if it's a bug or not.