AVR dev board , 2 MCUs , 2Power sources..?

Hey guys,
I'm trying to make an Avr development board
with an external lcd to use for debugging purposes
One chip will be programmed in a zif socket
I will break out the ports as well as separate place for SPI ICSP UART and TWI

How ever I do not want that lcd wired up to the MCU I'm programming because even in 4bit mode its using up the pins not to mention the avr programmer is also using up pins

So I want to have a separate chip probably a 328p or p-au driving the LCD and communicating with the chip being programmed via UART at most just listening which is just one wire and a com. Ground.
And i am going to have that lcd and the chip driving it running off an external power supply (not icsp) ..so my question is as follows:

Since the ICSP is going to give the chip being programmed power then the only thing I need between the two would be a common ground?

Or can I drive the them both off external power supply and just not connects the ICSP vcc to anything ..I don't think leaving that pin floating will be a good and I'm not sure if grounding it is right either. .I was hoping if I didn't have to power one of the microcontrollers via icsp then I can just program it then unplug the programer and it'll still be powered.

i am at work so here's a sketch of the idea..mind the penmanship and picaso like drawing .

Both chips will need to share the same gnd.