I know you asked to Rebase, but I'd Cherry-Pick the commits I wanted to move from Branch2 to Branch1 instead. That way, I wouldn't need to care about when which branch was created from master, and I'd have more control over the merging.
a -- b -- c <-- Master
\ \
\ d -- e -- f -- g <-- Branch1 (Cherry-Pick f & g)
\
f -- g <-- Branch2