It might happen, but I will be very disappointed if it does. I see no problem for the Brumbies nor the Wallabies if both Sio and Ala'alatoa remain on the LH side. Reserve props these days play almost as many minutes as he starters. It is imperative that every side has two good and reliable options on both sides. Up to the coaches of all teams to develop THPs who are up to the standard required, luckily with the help of Ledesma.
The worst move in the front row in recent history was Ben Alexander to the TH side. From battling with Benn Robinson for the starting LH spot, he certainly became one of the most capped THPs for Australia, but never to the satisfaction of the majority of fans and scribes. I wouldn't want to see a similar situation arise with either Sio or Ala'alatoa.