Only S10 Concept wheels can be converted from Single treadle to Double treadle. Just purchase a double treadle base for an S10 Concept. (partnumber SC0103)
Older S10's, S51's, S15's can not be converted with an S10 Concept base as the S10 Concept connection point between back post and base has a tapered connection for improved stability. This simply does not fit with older wheels.
How do you know if you have an S10 Concept or an older S10 wheel? The front of the mother of all, where the Louet logo appears, says "S10 Concept" on all S10 Concept wheels. If it just says "S10", it is not a concept wheel. See also S10 Concept - identification