Going with the make-up of the Explorers random encounter, here's what I've come up with for the other party:
The mage and the priest were brother and sister, Miri and Dravin Amblecrown, nobles from Berdusk. They hired a local scout, Mzolu, to be their guide, as well as six local tribal warriors to be porters. Their father is dying of the death curse, so they are seeking its end as well. However, Dravin stumbled upon a stegosaurus and got killed (the proof is in the six puncture wounds in his chest). Mzolu told Miri about the witch of Mbala and her magical powers of resurrection that seem to defy the death curse, so the group set out to find out if Mzolu's story was true.
Along the way, the group stumbled into some mad monkey mist, and Miri is still suffering some lingering after-effects from the fever, which have made her obsessed with raising her brother at all costs.
As an alternative to having this other group catch the PCs up as they head up the path to Mbala, I could make it so Miri is already visiting with Nanny Pu'pu when the PCs arrive. Mzolu asks them kindly to wait outside the hag's hut until his master emerges. When Miri comes out, having agreed to take on the pterafolk in exchange for Pu'pu's assistance, she attempts to convince the PCs to help her.
Since she's still a little loopy, if the PCs refuse, she orders her followers to attack them with the intention of capturing at least one of them alive to use as the living sacrifice. (She might also hope that the PCs have a 100gp gem for use with the ritual; if not, she will have one already - she is a noble, after all.)
If the PCs do help Miri get rid of the pterafolk, she instead attacks them on the way back to Mbala, still with the idea of capturing one of them to use as the sacrifice needed to bring her brother back to "life".
Because of Miri's lingering madness, it hasn't quite clicked that the hag's ritual will make her brother undead. The PCs can attempt to reason with her and convince her against going through with the ritual, but it will be difficult. If a PC uses lesser restoration (or similar magic) on her, then her mind clears and she can be more easily convinced of the craziness of her plan. (In this case, she could potentially become an ally of the party's - or maybe even a replacement PC, should one be needed.)