Is neutron-neutron fusion viable? Do I get unusual baryon maybe like pentaquark or just a pair of conjoined twin that is very unstable?
 A: 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.
A: Since this has been kicked up by community , I found this link:

The neutron–neutron fusion process,
    n
    n
    →
    d
    e
    ν
   , at very low neutron energies is studied in the framework of pionless effective field theory that incorporates dibaryon fields. The cross section and electron energy spectrum for this process are calculated up to next-to-leading order. We include the radiative corrections of 
    O
    (
    α
    )
   calculated for the one-body transition amplitude. The precision of our theoretical estimates is found to be governed essentially by the accuracy with which the empirical values of the neutron–neutron scattering length and effective range are currently known. Also discussed is the precision of theoretical estimates of the transition rates of related electroweak processes in few-nucleon systems.

Here is a reference to an experimental program to study $nn$ scattering, which data would be necessary to use in the theoretical study above.
