This is explained here: Multiple Association Relationship, so in your case you would do something like this for example. Note in this case, you must declare both sides of relations with properties.

in A:

<cf:property name="For" typeName="{0}.B" relationPropertyName="Back">
<cf:property name="For2" typeName="{0}.B" relationPropertyName="Back2">

in B:

<cf:property name="Back" typeName="{0}.A" relationPropertyName="For">
<cf:property name="Back2" typeName="{0}.A" relationPropertyName="For2">