I think the Raiders could play in the same stadium the 49ers do in Santa Clara, they just didn't want to before. If both the Rams and Chargers end up in LA though, the Raiders may not have much of a choice but to play in Santa Clara.
That would work in a last ditch short-term situation...but that can't be the full-time solution. Santa Clara is just too far from the (rapidly shrinking) fan base for it to work, the South Bay is dominated by San Francisco fans.
That and if you're on that side of San Jose it's really hard to keep the Oakland name.
The Raiders need a new stadium in Oakland for them to work out anywhere, really...and that's simply not on the table meaning they're in for a rocky decade no matter where they play. Even if they move 'back' to LA they'll be the
other team, the one that people with money won't go anywhere near. Their only hope of gaining the LA fan base at large was to be the only team here. With anything else around they'll simply be South-Central's team (regardless of the fact that both potential LA stadiums are in South-Central...imagine the Chargers being the primary tenant in a stadium in an area that would be 90% Raiders fans...sounds like a great idea, right?).
To compare LA geography to Pittsburgh...which is insanely difficult given the two regions are nothing alike...if the Staples Center (roughly the southern boundary of downtown) is Consol Energy Center, the Hollywood Park project (Rams; Inglewood, across from the Forum) would be around South Hills Village and the Carson project (Chargers/Raiders; near the StubHub Center) would be around Clairton. Except everything is flat, paved, and populated.
In Bay Area terms Levi's Stadium is in Uniontown.