Include the same forms twice in other one - Symfony2

I want to do something with Symfony but it seems to be complicate : I want to include the same form twice in other one. I explain: I have two entities named Evenement and TypeEvenement and on my form I want that the user select a TypeEvenement when he creates an Evenement or he can add a TypeEvenement.

Technically, it give this :

-> add ('typeEvenenement', 'entity', array(
'class'  => 'FFESponsorBundle:TypeEvenement',
'property' => 'nom',
'multiple' => true,
))

->add ('typeEvenement', 'collection', array(
'type' => new TypeEvenementType(),
'allow_add' => true,
'allow_delete' => true,
))

But there is a conflict. Indeed, the collection take the priority on the entity. I tried to modify my form like this :

-> add ('typeEven', 'entity', array(
'class'  => 'FFESponsorBundle:TypeEvenement',
'property' => 'nom',
'multiple' => true,
))

By define on my entity getters and setters for don't have error with the same values than getTypeEvenement(), etc ... But I have a gain a conflict. So I have tried to found on other solution : I have found the readonly property. By playing a bit with JQuery, I can put this property and in this case, I can create TypeEvenement object with the collection field. But when I want to do the opposite, the TypeEvenement selected is not joined to the Evenement created on the database.

I am a bit lost and I begin to think that's my problem is impossible.

Anyone has an idea about my 3 proposition or an other one to give to myself ?

Thanks guys