I was experimenting having a US and JPN X360 sharing a single live account and here's what I noticed (should be the same for Euro X360s as well).
--Profile can switch back and forth between two no problem. Keeps your gamerscore and friends list. Just have to recover the account each time by entering three lines of info (tag, email, password).
--If you purchase a xbox live arcade game on one system, you can download it on the other system free of charge.
--If you unlock achievement on one system it counts towards your total achievements....sort of. If for instance you play a Jpn game and get achievements it will go towards your gamerscore and when you go back to the US system the achievements will show up.
--But if you have the same game on both it freaks out. I got Geometry Wars on the JPN system and unlocked the first 100,000 achievement for 5 achievement points. Then I switched back to the US system and recovered the account and the 5 points were gone. But when I went into Geometry Wars it listed the 5 points as being unlocked, but then back out to the overall gamer score and look under geometery wars and it shows it as locked.
--So umm, the conclusion I've gathered is don't share Xbox Live Arcade games between systems. Pick one and play on it. I'm guessing this would happen with real games as well (like if you had RR6 on both systems). Otherwise profile switching between multiple systems seems to work without a hitch.
--Profile can switch back and forth between two no problem. Keeps your gamerscore and friends list. Just have to recover the account each time by entering three lines of info (tag, email, password).
--If you purchase a xbox live arcade game on one system, you can download it on the other system free of charge.
--If you unlock achievement on one system it counts towards your total achievements....sort of. If for instance you play a Jpn game and get achievements it will go towards your gamerscore and when you go back to the US system the achievements will show up.
--But if you have the same game on both it freaks out. I got Geometry Wars on the JPN system and unlocked the first 100,000 achievement for 5 achievement points. Then I switched back to the US system and recovered the account and the 5 points were gone. But when I went into Geometry Wars it listed the 5 points as being unlocked, but then back out to the overall gamer score and look under geometery wars and it shows it as locked.
--So umm, the conclusion I've gathered is don't share Xbox Live Arcade games between systems. Pick one and play on it. I'm guessing this would happen with real games as well (like if you had RR6 on both systems). Otherwise profile switching between multiple systems seems to work without a hitch.