According to Brent Ozar's blog (in the comments) it's 64GB per instance. But you should read his reply to that comment.
http://www.brentozar.com/archive/2010/06/sql-server-r-standard-supports-less-memory/[/url]
EDIT: You should also consider what will happen to the OS if you're using 2 instances that gobble up the rest of the server's RAM.