The Supplement DOES NOT state only one Plaything can be chosen per HQ. It clearly says:
"Any units from a Detachment or Formation presented in this book that can select Artefacts of Cruelty cannot select from those listed in Codex: Dark Eldar, but can instead select Diabolical Playthings, presented opposite, at the points costs shown."
Nothing about how many can be taken per HQ, or per army, only that only one OF EACH Plaything can be selected per army. The restriction of one per HQ therefore only applies to the Artefacts of Cruelty.