  Multi-particle sources constitute an interesting new paradigm following the
recent development of on-demand single-electron sources. Versatile devices can
be designed using several single-electron sources, possibly of different types,
coupled to the same quantum circuit. However, if combined \textit{non-locally}
to avoid cross-talk, the resulting architecture becomes very sensitive to
electronic decoherence. To circumvent this problem, we here analyse
two-particle sources that operate with several single-electron (or hole)
emitters attached in series to the same electronic waveguide. We demonstrate
how such a device can emit exactly two electrons without exciting unwanted
electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic
regime, perfect two-electron emission can be achieved by driving two quantum
dot levels across the Fermi level of the external reservoir. If a
single-electron source is combined with a source of holes, the emitted
particles can annihilate each other in a process which is governed by the
overlap of their wave functions. Importantly, the degree of annihilation can be
controlled by tuning the emission times, and the overlap can be determined by
measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel
experiment, the wave functions overlap close to the emitters and not after
propagating to the beam splitter, making the shot noise reduction less
susceptible to electronic decoherence.
