I would like to cite both Potter, Granger and Weasley (1995) and Potter, Granger and Weasley (1998) for the same claim. I would write
Hogwarts is a school of magic (Potter et al. 1995,1998).
Now let's assume I would like to cite both Potter, Granger and Weasley (1995) and Potter, Dumbledore and Snape (1997) for the same claim. Should I write
Hogwarts is a school of magic (Potter et al. 1995,1997).
or
Hogwarts is a school of magic (Potter et al. 1995; Potter et al. 1997).
I doubt it vary much among journals but FYI I am aiming Molecular Ecology who uses the APA reference style as indicated in the guidelines to authors.