Ok, just got of the phone with engineering. Here's the lowdown....
Firstly some technical stuff.
If you have more than one sim on the same account, only one can be active on the system at any given point in time. The last SIM that activates, will take control, disabling he first one.
Within the above constraints, TwinCall was developed, where you can have 2 SIMs on the same voice contract but only the last SIM will be active. If you turn both SIMS on, you get unpredictable results with SIMs turning on and off, seemingly randomly (as they re-acquire the network).
This worked fine for the intended application, for example having one mobile and one car phone with only one active at a time.
The same concept applies to data SIMs, only one can be active at the same time.
TwinCall on Data-only contracts was never (and still is not) a product you can buy over the counter. There was a technical anomaly in the systems that allowed data sims to work at the same time, but still with unpredictable results.
The answer was to take two or three sims but link them at billing level, not at network level. Now you can have two sims that allows you to share a bundle. This is called MultiSIM and is charged at a extra R9 per SIM.
Using MultiSIM is actually a bit of a jippo as the intended application was to have a phone and a datacard on the same contract.
So, you're not going to get any joy asking a retail shop for twincall on a data contract, they don't have it as a product.
I guess you could get a voice contract (4U?), get a twinsim on it and then strap a bundle to it.