How do I connect 2 HUs to 1 harness?

A friend wants to hook up 2 HUs in his car. I'm assuming to just splice the 2 HUs into the wiring harness to connect to the car's OEM harness.

1-------\
|---------------------X>
2-------/

Like such.

Is this correct?

[Honest Bob]

Why two hu's? I would get a wiring harness for the car
and start soldering. Only thing you cant do is have both hu's connected to the speakers. You will blow one or both units. You could use an rca output and get an adapter to put it into the input jack (if the HU has one) of the other head unit. I dont see the need for dual HU's though.

[Snausages01]

you sure he didnt just hook up the second hu to the first hus aux input ? i plan on doing something like that either with an XM reciever or a indash monitor wqith reciever.