  Magnetic reconnection and non-thermal particle distributions associated with
current-driven instabilities are investigated by means of resistive
magnetohydrodynamics (MHD) simulations combined with relativistic test particle
methods. We propose a system with two parallel, repelling current channels in
an initially force-free equilibrium, as a simplified representation of flux
ropes in a stellar magnetosphere. The current channels undergo a rotation and
separation on Alfv\'enic timescales, forming secondary islands and (up to
tearing unstable) current sheets in which non-thermal energy distributions are
expected to develop. Using the recently developed particle module of our
open-source grid-adaptive MPI-AMRVAC software, we simulate MHD evolution
combined with test particle treatments in MHD snapshots. We explore under which
plasma-$\beta$ conditions the fastest reconnection occurs in two-and-a-half
dimensional (2.5D) scenarios and in these settings test particles are evolved.
We quantify energy distributions, acceleration mechanisms, relativistic
corrections to the particle equations of motion and effects of resistivity in
magnetically dominated proton-electron plasmas. Due to large resistive electric
fields and indefinite acceleration of particles in the infinitely long current
channels, hard energy spectra are found in 2.5D configurations. Solutions to
these numerical artifacts are proposed for both 2.5D setups and future 3D work.
We discuss the magnetohydrodynamics of an additional kink instability in 3D
setups and the expected effects on energy distributions. The obtained results
hold as a proof-of-principle for test particle approaches in MHD simulations,
relevant to explore less idealised scenarios like solar flares and more exotic
astrophysical phenomena, like black hole flares, magnetar magnetospheres and
pulsar wind nebulae.
