Easily turn you Ultra Duo S2 into a travel system with these adapters, available in two options to suit your needs. Choose the one that's right for you:

Maxi-Cosi/Nuna/Cybex Adapter – Compatible with most US models of these brands.
Twin Adapter Kit – Includes one Maxi-Cosi/Nuna/Cybex adapter and one Universal adapter, allowing for dual car seat use. (For twin setups, one of each adapter type is needed, as two of the same won't work.)