If you describe a use case, we would be better able to offer suggestions.
According to the link I posted, you should be able to 'stack' them, by making multiple transactions. You will have to ensure enough credits are in your account to make multiple purchases, and you have to confirm (and complete) your purchase after each selection, then start over with a fresh purchase. There is no option to buy more than one of the same in a single transaction. If you are not able to buy more than one within a 30 day period, I suggest calling Telus to learn why their help page does not match the process you have experienced. You could also buy data 3 weeks before they arrive, and have it automatically renew halfway through their visit. This would give them 2 GB of data during their stay.
If you are purchasing data for a visitor who needs more than these provide, have a look at Public Mobile. You can buy as much as 6 GB of data for as little as $80. It has the same coverage as Telus, as it runs on their network.