In the full version there are Robot Voices and one I use for Megatron is Robot Voice One. It sounds like a deep voice but also there are musical tones kinda "dancing" around it like a second voice in it. I don't know how they did that. I would love to find a way to even create new voices myself. I have WinXP Pro SP2 now but programs for creating voices I hear are rather expensive.
I think to make a voice which is a combination of two voices, you'd have to make a completely new voice out of the two you want and use the new voice instead. I don't know if it's possible to use two voices at the same time as if it's one voice in any other fashion.