Cleaning/sanitizing isn't required, but it all depends on how risky you like to be! If the first batch wasn't contaminated and the second batch isn't contaminated, there's no additional risk of infection (above the standard risk that would come during racking whether you cleaned it or not). Assuming any leftover apfelwein in the keg won't screw up the next batch's flavors (for instance for beer, I wouldn't rack a pilsner into a keg that just had a stout) its certainly possible.
Of course the buzz kill in me does want to point out that seldom are batches completely free of other organisms. Normally they occur below taste thresholds and you don't have to worry about them, but if your first batch happened to be slightly infected, even if it didn't seem so, all those infecting bugs are still in the keg with a fresh batch to work on, potentially making their way to taste thresholds. Another problem (at least for beer, not sure about wine), is that when I finish a keg there's always a collection of yeast at the bottom that settled out. Leaving that in there means that now, rather than one batch's worth of yeast you've now got two, which means that it will take a little more volume to clear the yeast layer out of range of the diptube. I don't remember that much yeast in my apfelwein though, so this might not be a problem.
If you're the risky type (or at least non-paranoid type), feel free to go for it. Since I'm a little paranoid I'd probably go ahead and do a quick wash/sanitizing, but it all depends on how much your time is worth!