You can use shared neutrals, providing -
1. the two hot wires sharing a neutral are supplied from different panel busses, and
2. all the outlets supplied from those two circuits use the same neutral, if there is more than one neutral in the conduit.
You say "these circuits are already de-rated to 80%" Is this the "continuous load" derating, for Rule 8-104? If so, you only need to derate for that or Table 5C, whichever results in the largest wire size. 8-104(6)