Multiple carts with one authorize.net account

[DrByte]

The only potential problem I see is the default receipt URL - if the receipt URL is, for some odd reason, not passed properly, you may end up on the wrong site for your receipt.

Also, make sure you add both response URLs to your list too - otherwise, you'll have a site with rejected transactions.

If you're using the more reliable AIM module, instead of SIM, you don't have to worry about "receipt URL" or "response URL" settings ... because they're not used.