Why the need of another token though? Just to burn DEC first for a future discount?
And will this DEC-B be tradeable outside the game? If yes then it kind of defeats its purpose.
Why not utilize a weighted system where it allows someone to use all DEC or all SPS for payment with no discount or a mix of DEC / SPS / Vouchers for payment in equal weight where this provides a tiered discount based on the number of items purchased at one go.
You're more likely to reduce supply across the different items at one go if you incentivise utilizing the different exiting items already available rather than introducing yet another payment mechanism.