They're both called the "D" flag, and they both globally control the behavior of certain processor instructions. Besides that, they're totally different.

The 8086 D flag is the direction for string instructions, whether to increment or decrement the index registers after each repetition. One use was for moving memory between overlapping regions by starting at the end and working backwards.

The 6502 D flag is for decimal mode. When set, the instructions ADC and SBC do BCD arithmetic instead of binary, e.g. 0x09 + 0x01 = 0x10 instead of 0x0A as you'd expect. The 8086 also has support for BCD, but took a different approach, using separate instructions DAA/DAS for the same purpose.

Both can lead to bugs when you assume the flag is in the "normal" state but it somehow got flipped.