I understand how the GFCI works and how it should be wired, but in this
case, it's unimportant. The existing receptacle to be expanded to two
is downstream from the GFCI and protected by it.

My question is, when I install the two-gang box and the second
receptacle, do I need to pigtail the hot feeds to both outlets or can I
wire them in series (i.e. the hot/neutral from one receptacle feeds the
second receptacle)?