Example:
<confiugure>

<if condition="">
<propertyX>...</propertyX>
<then>
<appender name="FILE" ...>
<file>A.log</file>
</appender>
<then>
<else>
<appender name="FILE" ...>
<file>B.log</file>
</appender>
<else>
</if>
</configuration>

Support for nested ifs would be nice.