Add ons & Configurable options auto calculate quantity?

[QuidWeb]

I just got started with the Configurable options & addons with WHMCS however i am a little confused about something.

 

1. I have a product called "Enterprise" for example

2. I have a configurable option to allow the user to select a quantity, so for example 10 x £50 this works fine

3. I have a product add on called "Enterprise Support" for example, the intention is to allow the user to tick this add on and have the addon match the same quantity as the configurable option.

 

For example 10 x £50 for Enterprise & if Enterprise Support is selected add 10 x £5 giving a total of £550 + tax of course

 

Instead the addon is being calculated once so it is 10 x £50 + 1 x £5

How can i achieve this without asking the user to enter two quantities?

 

I do not want users selecting 10 Enterprise accounts and thinking they save money by adding only 5 support accounts, support is per user in this case.

 

ps. this is not a hosting product.

 

thanks for your help