Some times those double lines and no over taking signs are a plot they use on alternative routes to punish you or not taking the toll road. I recently went over Du Toits kloof pass and there were so many safe to pass spots with double white lines. Total BS.
The two roads fall within the jurisdiction of separate entities: the Huguenot Tunnel falls within SANRAL's juristiction, whereas the alternative Du Toitskloof Pass falls within the jurisdiction of the Western Cape Department of Transport and Public Works. Given the general antagonism between the two entities (see: Winelands Toll Project), the probability that the non-tolling entity would "punish" road users for not using the roads maintained by the tolling entity would be between minuscule and non-existent. As such, please provide more concrete evidence than mere hearsay for your claim.
The inverse does not appear to be true through: the quality of the SANRAL-controlled sections of the N1 and N2 (i.e. from the R300 onwards) is poor compared with the WCDOT-controlled sections (N1: between Koeberg Interchange and the R300; N2: between Hospital Bend and the R300), and this appears exacerbated since SANRAL lost the Winelands Toll Road court case. There is an opinion, shared by myself, that SANRAL wants to force a
fait accompli in an N1 and N2 being beyond repair and thereby forcing tolling.