I am in the process of replacing the connection from my car battery to my DC-DC. I am going from a 20A DC-DC to 40A and changing my cable to a 4 gauge instead of 8. The required length is approximately 13 feet.

I ran a new cable under the cad to connect the positive terminal.

My question is simple: can I use the two 8 gauge wires in place for the negative terminal?

8 AWG wire has cross section 8.37 mm^2. 4 AWG wire has cross section 21.2 mm^2. So two 8 AWG wires will not have as much copper conducting the current as one 4 AWG wire. In that sense, two 8 AWG wires are not a direct replacement for one 4 AWG wire.

However, in your particular application, if the round trip circuit length (battery positive terminal to DC-DC charger, then back to battery negative terminal) is 13 feet, then two 8 AWG wires will be more than enough to safely carry 40 A, according to this wire gauge chart: Blue Sea wire gauge chart
The chart shows that you would need one 6 AWG wire to carry 40A over a circuit length of 13 feet. 6 AWG wire has cross section 13.3 mm^2. So two 8 AWG wires, with combined cross section 16.7 mm^2, is more than enough.

But if the length of wire from the battery to the DC-DC charger is 13 feet, then the round trip circuit length is 26 feet, and the chart shows you would need 4 AWG all the way (for 3% voltage drop). In that case, your two 8 AWG wires would not be enough for the return from the DC-DC charger to the battery negative terminal, since their 16.7 mm^2 combined cross section is less than the required 21.2 mm^2.