Magento 1.9 extra discount on checkbox

I have been given a web shop site in Magento 1.9 and I need to implement extra discount on order if customer checks the checkbox. First problem is that one discount already exists and if I put another one programmatically, the second one overwrites the first one. I used http://ift.tt/1HawHDn extension for adding second discount, but when this one is active, it overwrites my first discount.

Second problem is that the second discount must be applied only if customer checks the checkbox. I'm not that experienced with Magento so I'm not quite sure where to begin.

For example:

Total: 100€

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Discount 1 (10%): 10€

Discount2 (5%): 5€ (applied only if checkbox is checked)

Total sum: 85€