Hello,
I am using FeynOnium to reduce the covariant structure to the Pauli-matrix form.
I can obtain the correct expression for a single fermion line (ubar3.u1) as shown below:

SpinorUBar[p3, mN] . SpinorU[p1, mN] // FMSpinorChainExplicit2

Next, I want to evaluate (ubar3.u1)(ubar4.u2) with two fermion lines.
I tried the following code, but the two Pauli sigma matrices are not distinguished:

SpinorUBar[p3, mN] . SpinorU[p1, mN] *
SpinorUBar[p4, mN] . SpinorU[p2, mN] //
  FMSpinorChainExplicit2//Expand

Could you help me with this problem? Thanks a lot !
BTW, I notice that the current stable version of FeynOnium is not compatible with the FeynCalc 10.x.