Ok, i see what's going on here. But first i want to apologize to you fps. You must have been real confused when i asked an incoherent question, and when you tried to help i jumped down your throat. so for that im sorry.
tldr: it isn't supported in hexxit.
I've played tinkers construct in other modpacks and stand alone, and when you click the molten metals in the controler they would switch places with the bottom layer. This would allow you to chose which metal you want to pour into a cast without draining the bottom layer off first.
so there it is. Thanks for the help fps, i appreciate it.