definitely remove. first, if you didn't how would you attach the new
siding? it could be done, but it would add a lot of work to the job
that could be spent removing the stucco. and how about windows and
doors? if you added a layer of siding, they probably would have to be
built out. and it is possible there has been water intrusion if your
stucco is in bad shape; you'll want to repair any damaged areas. all
that said, removing stucco is no cake walk! good luck!