And of course, it was simple and dumb.... This
driverClass="net.sourceforge.jtds.jdbcx.JtdsDataSource"
should have been this
driverClassName="net.sourceforge.jtds.jdbcx.JtdsDataSource"
I'm guessing it worked with the second resource because the first had already loaded the driver into the classpath (They run on the same tomcat instance).
That was frustrating :)