As to your question on splitting power and ignition sense - you simply connect both HUs to the wires in the car. This might cause some problems though, depending on how beefy the wires are - you might have to run a new heavier gauge constant +12V wire from the battery to reliably power both units. Same goes for the ground wire - it might be insufficient, but that one only needs to go to a solid piece of metal that's part of the cars metal shell (some have been fooled by relatively large pieces of metal embedded in the dash...) not the battery.

Two HUs and an amp shouldn't scare an installer - I had a JVC radio-cassette player feeding into the AUX in on the empeg for a while, two amps (mids/highs and sub) without any problem. (But I do consider myself not totally lost around electricity...)
The main pain with installations are, IMHO, the mechanical disassembly required to get the wires routed...

/Michael