1. Yes - share the same database. Not too difficult providing your sites are only design different (look and feel) and not structurally (categories and extrafields different).
2. Easy to have two languages but you will need two different systems (unless their is a module in
ClassifiedMods - have not checked recently). You need to be clear whether you expect the advertiser to enter the advert in two different languages or not. If not, then someone will need to translate.
Google will do translation of your site for you by the way and it avoids having to have two installs as well. The qulity is variable.
I think from an
SEO perspective, mutliple sites with identical content is not such a good idea. You will arrive with a lot of duplicate content. Your sites may get penalised for it.