Yes. Or vice versa. Both the caster and the spell are treated as a single model and there is no rule saying a single model cannot cast either endless spell. There is also no rule creating or addressing an "attached" status. More notably, if you cast the cauldron first both it and the wizard would jump onto the vortex whereas if you cast it second it would be next to the vortex creating a larger footprint to be measured from. There is tactical value in deciding which way to go with.
I could definitely see them FAQ/Errata-ing this because RAI may be different.