would it work to pitch new wort on top of a fresh yeast cake from a carboy that just got done with secondary? or should i be using the stuff that is on the bottom at the end of primary?
It depends on how much yeast is there, what the first beer was and what the second one will be. Typically, if pitching a normal gravity ale on top of a fresh cake in a primary, only about 20-30% of the cake is needed.
There will be some effects if you re-pitch that yeast multiple times (if you use it once, the effect will be lessened). The primary will tend to have the more floculent yeast settle, the secondary will be the less floculent yeast.
if OG is higher, great because you have more yeast. Remember some flavors and color could carry on to the 2nd batch. Ale to stout is not a color issue but hop trub might contribute to some interesting flavors. I would add sanitized water, swirl it up and pour off the top yeast portion into a clean carboy to put the new wort on. It's like a fresh start with a big starter.
would it work to pitch new wort on top of a fresh yeast cake from a carboy that just got done with secondary? or should i be using the stuff that is on the bottom at the end of primary?
i have thought about yeast washing, but i just don't have a lot of space to store it right now so im holding off on that. most of the beers my make are more on the malty side and less hops so i think the hops in the trub wont be much of an issue, so the cake that i want would be after primary.