Preamp out on amp heads

[Warheart]

If you wanted to set up two amp heads, but only use the preamp of one of them, what kind of cable should you use to go from the preamp out from head 1 into the FX in(or preamp in) of head 2? like, speaker cable, or guitar cable?
i know theres always an A/B box option , but i just need to know what kind of cable to use for preamp out applications.
Thanks.

[some_dude]

Re: Preamp out on amp heads

Shielded insturment cable.

The same stuff you use to plug in your guitar.