Place potato and kumara in a single layer, in a large steaming basket over a pan of simmering water (don't let base of steamer touch the water, cover with a lid or foil and steam vegetables for 12 to 15 minutes or until tender. (I would use my electric 3 tier steamer but if you can't steam I don't see why you can't boil in water till just tender and drain well).
Remove from heat, drain and transfer to a large bowl to cool.
Meanwhile whisk sour cream, mayonnaise, gherkins, dill, mustard and juice in a small bowl.
Combine potato, kumara, sour cream mixture and egg in a large bowl.