...

A longer sequence offers more scope:

  a := b + c
  d := e - f

Now we can do somewhat better.

LW      Rb, B         LW      Rb, b
LW      Rc, c         LW      Rc, c
ADD     Ra, Rb, Rc    LW      Re, e
SW      a   Ra        ADD     Ra, Rb, Rc
LW      Re, e         LW      Rf, f
LW      Rf, f         SW      a, Ra
SUB     Rd, Re, Rf    SUB     Rd, Re, Rf
SW      d, Rd         SW      d, Rd

two stalls            no stalls

Notice that for this to work we need to have plenty of registers (how many extra?)