In relation to the B1.2 the only difference in knowledge requirements to a B1.1 is that you do the pistons instead of gas turbine, so you'd need the pistons as well, as you say.
However you'll have to do 6 months experience. It's all outlined in part 66. See the link below.
Pages:
L315/82 to L315/83 for the knowledge requirements
and
L315/144 for the conversion experience requirements.
http://eur-lex.europa.eu/LexUriServ/...en00010165.pdf