Do I get unusual baryon maybe like pentaquark or just a pair of conjoined twin that is very unstable?
Under isospin symmetry, the dineutron should be a "mirror nucleus" with the diproton and the spin-zero deuteron. Neither of those are bound (the deuteron has spin $\hbar$, and no stable excited states), and so there's no stable dineutron to fuse into.
Stipe Galic points out the possiblity of the weak interaction process $$ \rm n + n \to d + e^- + \bar\nu $$ as the isospin analogue to the proton-proton reaction in the core of the Sun, $$ \rm p + p \to d + e^+ + \nu $$ The core of the Sun is dense hydrogen under enormous pressure with a power density of about $100\rm\,W/m^3$; I'll let you work out for yourself the (in)feasibility of observing neutron-neutron fusion under terrestrial conditions.
High-energy neutron-neutron collisions will excite the baryon-meson spectrum in the same way as high-energy proton-proton collisions, but it's hard to make high-energy free neutrons and there aren't pure neutron targets.