Posted this on the idea before I saw the discussion:
You can do this by using "+" after the first part of your email address. This is how I use a single address for my multiple client instances. Examples:
Marketo A
User ID: abc+1@xyz.com
Marketo B
User ID: abc+2@xyz.com
etc.
Almost all major email clients recognize an email address with "+xxxx" as whatever precedes the "+"
"abc+1@xyz.com", "abc+2@xyz.com", and "abc+3@xyz.com" will all default as "abc@xyz.com".
You can also use this tactic to test forms/campaigns using the same email address. Hope this helps!