What you need to do is have a single <logger> definition with a defined level of INFO, but in your two appender definitions, you set their thresholds accordingly, e.g.
<appender name="ERROR_FILE">
<param name="Threshold" value="ERROR"/>
</appender>
<appender name="GENERAL">
<param name="Threshold" value="INFO"/>
</appender>
You then add both appenders to your logger:
<logger name="com.acme">
<level value="INFO"/>
<appender-ref ref="ERROR_FILE"/>
<appender-ref ref="GENERAL"/>
</logger>
Log entries now going to the logger will get sent to both appenders, but since they have different independent thresholds, the ERROR_FILE appender will only log ERROR and above.