I initially bought an extra JTAGMkII/STK500 with the recent $150 bundle price with the intention of selling the additional JTAGMkII and keeping the STK500.
However, I recently started work on a PS/2 to RS-232 8-pin interface chip based on a Tiny45/85. I created a test suite using another AVR to emulate the keyboard to test the correct operation of the PS/2 to RS-232 interface chip.
I'm pleased to report that simultaneous use of two JTAGMkII's to debug both the keyboard host (PS/2 interface Tiny 45/85 chip) and the emulated keyboard (AVR-M128 development board by Olimex) has worked very well both with AVR Studio and IAR's AVR compiler/debugger. It's been quite useful to have simultanous debugging environments (debugWIRE for the ATTiny85 and JTAG for the Mega128) for both the device under construction and the tester MCU.
Happily, I'll be taking the extra JTAGMkII off the sell shelf