On our current site, a late 90's PERL roll yer own, this feature was implemented as a separate SKU. Of course this results in confused customers trying to buy their 2 and having 6 arrive!
My preference would be that someone buys qty 2, we add a line item at no charge.
qty desc tot
2 DigStim $20.00
1 DigStim $ 0.00
Something tells me this is going to require a little more logic than the B1G1 would since there is a qty threshold test to meet before feature kick off.