There have been times where I have had a batch ferment in the primary. I will rack the first batch into the keg, then put a 2nd batch directly on top of all the settled out yeast. I think it is advantagous to do that as fermentation starts within hours and you have less chance for contamination. I haven't experienced any off flavors. I don't let either batch stay in the primary for more than 7-10d, so I haven't had a problem with off flavors from decomposing yeast.