On Indy the CMs will actually point out that the single ride slip says you will be separated. The loading system there is terrible, the cars often leave with empty seats because the CMs can't slot people in as fast as the ride loads. (It may also be a weight distribution thing on the empty seats, but since the cars pull forward before half the riders have seat belts on, I tend to think it's speed.) Then in the rush the far side platform almost never pulls from the SR holding area, and can be so many SRs waiting they'll suspending handing out passes. Forget fixing the eyes of Mara or the fires, they need to fix the way the ride loads. Anyway, I've been an SR on that ride with an empty seat next to me while other SRs wait, but I've rarely seen 2 be allowed in the same car.
On Matterhorn, I did end up with a friend once in the same car, but basically a party of 10 from a tour barreled through the exit, pushed aside a the GAC party waiting with a wheelchair and immediately started hassling the CM, so the CM cleared out the SR queue to have somewhere to put them. Another time my sister and I were split, then the kid behind me ended up in the other car paired with mine. But for the 2 times that's happened, I'd say there's been 4 times where I've gotten off the ride to the person behind me still waiting to ride.
On Splash I have never seen 2 SRs ride together.
On Goofy's it's pretty unlikely because there would have to be 2 single riders that went through the line together for there to be 2 empty seats in to vehicle.
On Screamin you might be on the same train, but like Indy and Matterhorn the CMs would rather pull a party of 2 from the middle of the line than go to the SR line to fill spots.
I haven't done Soarin enough to know your chances, it's a horrible ride for SR. They hold you around the corner where the ride loaders can't see you, and by the time they call you over the ride may be loaded and you have to go back. I didn't feel like it saved any time the times I've done it, so I stopped. And I've never done GRR, so no advice there.
RSR is probably the best chance that 2 SRs have to ride together, though it's just as likely to be the stranger in front of you instead of your friend behind you. At the very least there are usually 2-3 SRs in each batch of 4 cars, so you're at least likely to be on the ride at the same time or in quick succession.