How to set Bash completion with more a function

[Budi]

How Bash' complete the second argument using another different
complete's specification/definition and function than the first one
e.g.

complete func foo

Inside the function func must perform detection if it's at 2n
argument, the complete's specification/definition must be by the other
one e.g.

complete functn bar

and such the rest inside the function functn