Declaring objects

following problem
in the project i have a reference to 2 dll's which each have a function e.g.
dll_one with function tst_one
dll_two with function tst_two

now in my main app i have to following sub

Start_obj(byval sName as string)

I want to use either tst_one or tst_two, depending on sName which is either
"dll_one" or "dll_two"

normally I would use it as follows:

dim oFunc as new object

oFunc=dll_one.dll_one(or two)

oFunc.tst_one (or two)

How do i define oFunc using sName?