Discounting A Product when bought with another

[wemblystiggs]

Hi,

What I would like to do is offer a discount when you buy an accessory that can be used with the main item.

I.e.

You can buy E-stim boxes - £300
You can buy E-stim Electrode - £75

But when you buy them in the same transaction you will get 10% off the electrode and pay the full price for the box.

Any ideas?