Distance between master and slave

[Hammer]

Hi, an thinking about upgrading my amps to D800's.  My speakers are roughly 18 feet apart and I wonder if that is too far or will in need to invest in longer speaker cables in order to let the two amps sit closer together.  Any thoughts?  There seems to be a lot of discussion on the right cable to use for optimal sound, but I've not seen much discussion on the maximum length before clocks/timing is affected.

Thank you.  Hammer