Combine two columns in SPARQL

[Mikael Pesonen]

Not Jena question but hope someone can help. I have two columns with 
always equal amount of rows. How can they be combined into one column 
(variable)? This method doesn't work (example has different predicates):

select ?s ?pl_al
where {
    ?s skos:prefLabel ?pl .
    ?s skos:altLabel ?al .
    bind(concat(?pl, ?al) as ?pl_al)
  }