If you only have two, I would suggest one at a station on the Belair line and one at a Gawler line station between Greenfields and Salisbury.
The Belair line gets you Adelaide-Melbourne freight, GWA grain trains and daily Overland services on the standard gauge line plus DEMU metro passenger trains on the broad gauge.
Putting it further up in the hills after Belair would "filter out" the DEMUs - perhaps you could approach the private owners of the former Mount Lofty Station at Stirling which operates as a B&B and has the occasional short service or track machines stop on the short passing loop there. It's at the "summit" of the line so all trains are moving slowly at that point, and being a fairly long and steep incline to get there guarantees a good mix of locos, especially if GWA have to get their old ones out to boost the numbers on grain trains.
I believe the former Aldgate Station is also now privately owned (or council owned, not sure) and used as a community centre, they could be worth talking to if Mt Lofty doesn't come through.
A Gawler line station would be more difficult as all the stations are owned and operated by the state Department of Planning, Transport and Infrastructure (operator of metro trains and stations) but the reward would be a mix of Adelaide-Perth and Adelaide-Darwin freight, the Indian Pacific and Ghan on the standard gauge, plus the Penrice stone trains and Gawler line DEMU services on the broad gauge. It could also have a double function as an "extra" security camera which might be enticing for DPTI and could allow it to be mounted on the same tall poles the security cameras are on.
Keswick would be a waste. The interstate running lines are on the opposite side of the interstate passenger terminal to where the passenger platforms are, so you would be restricted to seeing one or the other.