Nobody in particular, it was what is called "folklore". The idea came up naturally when muon decay was observed by several groups in 1948, see Anicin's The Neutrino - Its Past, Present and Future:
"When in 1948
the electron spectrum from muon decay was found to be continuous it became
obvious that not one but two neutrinos are emitted along with the electron.
Pontecorvo witnesses that at that time everybody felt that the two neutrinos
should be different. They were even named differently, the "neutrino" and the
"neutretto", but with time the idea seem to have been forgotten and it was only
in 1962, when the difference between the two neutrinos has been clearly
demonstrated in the first of a long series of important accelerator neutrino
experiments, that the electron and the muon neutrino were finally given life."
The Pontecorvo reference is to his The infancy and youth of neutrino physics: some recollections, where we read:
"Several groups, among which J.
Steinberger, E. Hincks and I, and others were investigating the (cosmic) muon decay.
The result of the investigations was that the decaying muon emits 3 particles: one
electron (this we found by measuring the electron bremsstrahlung) and two neutral
particles, which were called by various people in different ways: two neutrinos,
neutrino and neutretto, $\nu$ and $\nu'$, etc. I am saying this to make clear that for people
working on muons in the old times, the question about different types of neutrinos
has always been present. True, later on many theoreticians forgot all about it, and
some of them "invented" again the two neutrinos (for example M. Markov), but for
people like Bernardini, Steinberger, Hincks and me ... the two neutrino question was
never forgotten... How to perform the decisive experiment I was able to formulate
/40/ clearly enough (the use of muon neutrino beams). At the time the idea of
the experiment was not obvious, although the statement may be strange today: one
must search for electrons and muons produced in matter by muon neutrinos".
/40/ is the reference to Pontecorvo's 1959 paper in JETP, The Universal Fermi Interaction and Astrophysics.