Weather-wise, June & July are not the best months to be visiting Santiago and Buenos Aires. The days will be short (about ten hours of daylight) and temperatures will be cool, probably around 15C during the day and in Santiago temperatures will likely dip to near freezing at night (it will be a few degrees warmer at night in BsAs). You could get rain in either city. In my experience it rains more often in the summer in BsAs but winter is the rainy season in Santiago and you could get a day-long storm easily enough. The benefit of this is that this will clean the air (see below) and you will be rewarded with a spectacular but short-lived view of the Andean cordillera immediately east of the city.
Santiago has mad significant strides in improving air quality over the years but it remains prone to stable thermal inversions in the winter which bring choking smog. There isn't much you can do about other than hope for rain, and be thankful that you don't have to breathe that stuff all winter long.