Right, so, the rulebook says:
"when a FC comes into play, it is placed directly on top of th existing Payer Card. A Player may only use a FC containing the name of his/her Player Card. A FC can be discarded at any time."
There is no bit, like about equipment, for playing forms during the action phase. From this text, I am presuming that there is nothing wrong with stacking forms. If I've already got Wisdom Form on my Sora, there's no reason I shouldn't be able to then put a Valor Form on top of that, with the Wisdom Form coming back whenever I choose to discard the valor form(or if I lose a challenge). Alternately, you could slap a Wisdom on top of a Valor to protect it from challenge losses, then take off the Wisdom when it suits you...