This is a great question. Let me try and answer as fully as possible.
Firstly, it's important to remember that Double Data and 2 for 1 Topups are two different promotions. Double Data refers to getting double your monthly data allocation on either a Capped DSL account or Mobile Data. So if your package is 2GB per month, you would receive 2GB + 2GB FREE while the promo is running or while you have Afrihost Plus activated.
2 for 1 refers to Top Up data, when you run out of available data or want to purchase a little extra just for this month. When you purchase any amount of data, you receive the same amount on top for free. So a 2GB Topup becomes a 2GB + 2GB Free Topup (which expires are the end of the month) at the same price.
On Afrihost Plus+, you can assign these benefits to either a Capped DSL account or a Mobile Data account. You can also have different benefits on different products, so you could have double data on your DSL account and 2 for 1 topups on your mobile product- it's entirely up to you to customise to suit your needs (and wallet).
You also get a 30 day free trial to try out Plus+ membership.