Planning to go to both cities. Flight is booked. Any tips? Must sees in either city?
some spanish words mean different things in different south-american countries, trust me I come from one of those. 
My D did a graffiti tour in Buenos Aires that she loved.
Have fun.
If you are going to say that, it would be actually be helpful to give examples of such words, to be useful to mstee.
July, as I’m sure you know, is winter down there. Make sure you pack some layers. Central heating is much more rare in BA and Santiago than in the USA and some places can be chilly.
BA - definitely take in some Tango or try a lesson if you are up to it.
In Chile, try to pop over to Valparaiso and take in the mural culture and some seafood. In Santiago, visit the Museo de la Memoria y los Derechos Humanos, Human Rights museum focusing on remembrance of the period under the Pinochet regime.
Its been awhile since I was in Buenos Aires. It is an incredible city. First of all, it is huge! The food is great especially if you like steak and red wine. Not a hot spot for vegetarians! La Boca is a colorful neighborhood but be careful there and stay in the tourist area. When we went there the police stopped tourists from going in too far. I think what I enjoyed the most was just walking around and sitting in cafes. Be aware that locals don’t go for dinner until after 9 pm but since we were not local, we went earlier. Recolata Cemetery is interesting to walk around. We stayed a bed and breakfast and it was wonderful. We had no problems getting around but many people did not speak English and the Spanish they speak is Castellano. I would suggest googling and seeing the difference. Oh, and eat some empanadas. Argentines make great ones! And if you can, fly out to Iguazu Falls- beyond incredible.