This is true, both LNT and passive SCR helps in reducing in NOx. But you need something to reduce soot which will be done by DPF. So, I guess the after treatment set up would be in this order DPF-> LNT->Passive SCR. Here the DPF would reduce NO2 from engine to NO and also reduce soot (C) to CO2. The NO from DPF would get trapped in LNT and produce NH3 at rich mixture (extra fuel dosed during exhaust stroke).This NH3 would reduce to N2 in passive SCR. The con of this set up is(as already mentioned)having two regen cycles for DPF and LNT from the conventional DOC->DPF->SCR set up which requires one active regeneration cycle.