Generally speaking, "jacking under the chassis rail" is where these Audi jacking point protectors are located front and rear. Jacking under the rear beam, then is just a torsion bar made out of a folded section of steel, never jack under that unless your next new thread is "
how to reduce rear positive camber". Jacking under the ends of the beam, where the hubs get mounted on is possible but is not horizontal - which could lead to other issues.
Needing to jack up both sides at once if using Audi jacking point protectors, never heard of that, though if you have the cash, jacking one side up at a time using two jacks must be kinder to the car's chassis long term. Where things might have gone wrong when using these Audi jacking point protectors is due to the fact that all VAG cars have these 4-off locations in the chassis, and some people, mainly in USA, have fitted them to Passat W8 etc and that area of the chassis has deformed when only lifting at one point - that is not an issue with these smaller/lighter cars.
Your ideal trolley jack should have a "cup" on the top of the lifting saddle, and these Audi rubber bits fit inside the cup without bottoming and letting the top edges of the cup touch the chassis. Maybe try Machine Mart for trolley jacks and give Halfords a miss if possible.
The big things to get right when picking a trolley jack is, "lifting capacity" for small cars 2Tonnes should be okay + "closed height" + "extended height", the closed height needs to be low enough to get under the car at any jacking points, you can measure this on your car, the extended height will be the height you need to lift to to get something like axle stands under your chosen location - Jack Sealey brand is very good for DIYers I've found and long chassis (FRAME) jacks are quite stable.
Like any other lifting points on any car, they are mainly meant to be used with workshop lifts, which lift the complete car in one go, though remember some areas also need to be able to cope with changing a wheel at the side of the road with a simple single point lifting device.
