By performing an inc DPS, this selects whether DPTR is pointing at DPH0/DPL0 or DPH1/DPL1. Thus, without using the DPTR reference, one must explicitly address DPHx/DPLx. Please see page 6-19 of the EZ-USB Series 2100 Technical Reference Manual for the normal usage of the dual data pointers.
The steering via DPS occurs only for instructions that access the whole DPTR, not for instructions that access halves of the DPTR register (as you were trying to do). So, a MOV DPTR,#8000H could go into either DPTR1 or DPTR0, depending on the state of DPS, while a reference to DPH or DPL always means DPH0 or DPL0.