I think this is possible. You can just add a second number generator into the OIR
<!-- set the number to a generated number -->
<AttrValue id="number" algorithm="com.ptc.windchill.enterprise.revisionControlled.server.impl.NumberGenerator">
<!-- add a V prefix for variant parts -->
<Value algorithm="wt.rule.algorithm.BooleanBranch">
<Value algorithm="wt.rule.algorithm.EqualsTest">
<Attr id="genericType"/>
<Arg>variant</Arg>
</Value>
<Arg>V</Arg>
<Arg/>
</Value>
<!-- the sequence -->
<Arg>{GEN:wt.enterprise.SequenceGenerator:number_seq1:7:0}</Arg>
<Arg>{GEN:wt.enterprise.SequenceGenerator:number_seq2:7:0}</Arg>
</AttrValue>