Pedantic note: if housemate-to-housemate transmission is 30%, then there’s a 30% chance that you infect each of your two housemates, which indeed gives 0.6 sick housemates—but in the case where you infected exactly one of them (0.42 of the time) there’s then a 30% chance that they infect the other one after all, giving an extra 0.126 sick housemates for a total of 0.726.
(Well, maybe. Perhaps that 30% figure is partly because some people are just harder to infect, so that conditional on your having infected A but not B, B is then less likely to get it from A.)
Pedantic note: if housemate-to-housemate transmission is 30%, then there’s a 30% chance that you infect each of your two housemates, which indeed gives 0.6 sick housemates—but in the case where you infected exactly one of them (0.42 of the time) there’s then a 30% chance that they infect the other one after all, giving an extra 0.126 sick housemates for a total of 0.726.
(Well, maybe. Perhaps that 30% figure is partly because some people are just harder to infect, so that conditional on your having infected A but not B, B is then less likely to get it from A.)
Pedantry appreciated; you are quite right!