How to program multiple MCUs on a single PCB?

Here is the situation:
on a single PCB there is 1 main MCU, connected via SPI to 8 slave MCUs. Slave MCUs are ATxmega series. Main one can be anything; for simplicity and uniformity I plan to use same ATxmega. Main MCU will have USB interface, so I can program it via USB (? not sure about this; before I used UART to program MCU, don't know if I can do this via USB). But how to program those 8 slave MCUs? Program will be absolutely the same for each of them. Can I just parallel UART pins from all of them and run programmator? Or I somehow can program those slaves via SPI using master one? I just have no idea how to program them with minimal parts and efforts (since I hope this device eventually will be mass-produced ).

Second question - does anybody have link/example of absolutely minimum schematic to use ATXMEGA8E5? And if any of those parts can be shared between multiple MCUs on the same PCB?

Search in google "bootloader"

Hello!
I have done that kind of experiment (i.e. programming an MCU from another MCU. There are different approaches.
I did it by JTAG (Use the first processor to program the target by JTAG), and I also did it using a bootloader strategy
ass suggested by Easyrider.
In case of using the bootloader approach, you have to either modify the default bootloader, or create your own bootloader
in program flash. I choosed the latter because the default bootloader may be useful in case of a catastrophy: you can
still upgrade your program with standard means.
That said, I have never used ATMEGA, so I cannot give you accurate hints.
Dora.