1 Item with two selectable options (which are also products)

[mediaslave]

A wierd one but please hear me out:
I have a product which is a guitar fx pedal. It is a shell as such. You buy this and get 2 'modules' with it. You can choose 2 of 4 existing options. Each module can also be purchase seperately so is also a product.

What I would like to achieve is that the main pedal (the shell) is a product, each module is a product and when you purchase this combination you choose the main pedal and select 2 of the four modules. This will already be a win for me.

Additional I would like the person to be able to select one or two of the remaining modules options but this will then increase the price per module.