  In this paper we aim to push the analogy between thermodynamics and quantum
resource theories one step further. Previous inspirations were based
predominantly on thermodynamic considerations concerning scenarios with a
single heat bath, neglecting an important part of thermodynamics that studies
heat engines operating between two baths at different temperatures. Here, we
investigate the performance of resource engines, which replace the access to
two heat baths at different temperatures with two arbitrary constraints on
state transformations. The idea is to imitate the action of a two--stroke heat
engine, where the system is sent to two agents (Alice and Bob) in turns, and
they can transform it using their constrained sets of free operations. We raise
and address several questions, including whether or not a resource engine can
generate a full set of quantum operations or all possible state
transformations, and how many strokes are needed for that. We also explain how
the resource engine picture provides a natural way to fuse two or more resource
theories, and we discuss in detail the fusion of two resource theories of
thermodynamics with two different temperatures, and two resource theories of
coherence with respect to two different bases.
