There is no reason to duo grind together, expect the designated group grind spots, which require a lot of gear. 

Black Desert is an MMO after all, and new players are punished by the taxes to special deals. If they decide to grind together at grind spots not designed to be grinded in a party. The tax on special deal used to be a lot less then what it currently is. Why increase it? Should new players really have to grind alone untill they can reach miru(lol)/gyfin(dead)/Padix island/turo/abandoned monestary.

Is the tax still 15%? or has it seen a change?

Special deals gives you the base price of an item and divide it with all party members, so for example a caphras stone has a base price of 900k, so if you're 3 people in party you get 300k each. Which is a tragically bad deal. If it was determined by what items are sold for, for example a caphras stone on EU right now has an average price of 2,350,00. Then divide it on 3 party members, each would get 783,333 silver. 

Getting rare items when using special deals is even worse.

Each person doesn't get 783,333 silver because you forgot the 35% tax.  Each of those people now get 509,166 silver after tax.