Yes, because they are both extremely famous and they both already have a lot of big hits. They are both common household names with today's younger generation. Miley Cyrus is especially good at catching media attention. Even if the song isn't very good, it will still become popular because of all of the media attention they both receive.
Yes, the new song "Don't Dream It's Over" will top music charts in 2015. Ariana Grande is an impressive new and upcoming artist. When paired with the publicity Miley Cyrus has and will continue to gain, I feel that it'll be played on repeat on all major pop radio stations. Miley Cyrus and Ariana Grande are both very talented vocally and will create a wonderful duet.
Sadly, I believe this could reach high on the charts. Miley and Ariana's voices aren't really suited to this song (or anything in my opinion, they both have terrible voices but that's another issue.) However people often lap up anything by these big name people, they have rabid followers. So while their version seems to be completely devoid of the emotion and humanity of the original song I think their version could likely make a mark on the charts.
This awful cover version shouldn't be allowed in the charts, never mind topping them. It can hardly be called a new song. Although Miley Cyrus has never been the most impressive performer, at least before this she has produced original music, this cover is awful and a massive step backwards for her. Hopefully it will sink like a stone so she's not encouraged to do more!