These units are made of two Skaven models mounted on a single cavalry base. Treat them exactly like cavalry models. This means that they have a Unit Strength of 2 and get a 6+ basic Armour save (to represent the fact that there are effectively two models to kill, similar to cavalry models). This also explains their two Attacks. The only difference from normal cavalry is that they cannot be singled out as targets by enemy shooters because they are the same size as the other Skaven around them.