Finding U2 using superposition for this op-amp
You computed the load on U2 not the attenuation ratio. Also for practical reasons all R’s  should have K after the ohm value.  so try again using the methods of mesh or Norton Transforms or whatever.

1) separate load from source
2) compute equivalent cct of source from left side divider ratio Veq=1/2*U2, Req=  R2//R3
3) compute V(R5) across R5 in next diagram 

e.g.