Ok why dont you rename srcip and source-address to other variable like
sourcetype=source1 sourcetype=source2|eval srcIpAddress=if(isnotnull(srcip),srcip,if(isnotnull(source-address),source-address,NULL))|lookup Lookup.csv Address as srcIpAddress
Or you can join it twice like below
But make sure that you will take backup of Time,Attack fields before 2nd join (Because they will be replace in the second join)
sourcetype=source1 sourcetype=source2|lookup Lookup.csv Address as srcip|rename Time as temp_Time,Attack as temp_Attack|lookup Lookup.csv Address as source-address|eval Attack=if(isnotnull(temp_Attack),temp_Attack,Attack)|eval Attack=if(isnotnull(temp_Time),temp_Time,Time)
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