Запрос 1 (работает, выбираем assignedagent):
select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin, cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype, u.userlastname as assignedagentname , "" as answeringagentname from callrecord cr left join (agentrecord ar left join users u on ar.agentkey=u.userkey) on (cr.callid=ar.callid and cr. assignedagent =ar.sequencenumber);

Запрос 2 (работает, выбираем answeringagent):
select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin, cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype, "" as assignedagentname, u.userlastname as answeringagentname from callrecord cr left join (agentrecord ar left join users u on ar.agentkey=u.userkey) on (cr.callid=ar.callid and cr. answeringagent =ar.sequencenumber);

Запрос 3 (ошибка, выбираем оба):
select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin, cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype, u.userlastname as assignedagentname , u.userlastname as answeringagentname from callrecord cr left join (agentrecord ar left join users u on ar.agentkey=u.userkey) on (cr.callid=ar.callid and cr. assignedagent =ar.sequencenumber) left join (agentrecord ar left join users u on ar.agentkey=u.userkey) on (cr.callid=ar.callid and cr. answeringagent =ar.sequencenumber);

Подскажите, как правильно в варианте 3 нужно составить запрос?

Вроде нагуглил похожее решение:
select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin, cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype, u1.userlastname as assignedagentname, u2.userlastname as answeringagentname from callrecord cr left join (agentrecord ar left join users u1 on ar.agentkey=u1.userkey) on (cr.callid=ar.callid and cr.assignedagent=ar.sequencenumber) left join (agentrecord ar left join users u2 on ar.agentkey=u2.userkey) on (cr.callid=ar.callid and cr.answeringagent=ar.sequencenumber);
Но всё равно ошибка.

buser , в этом и было дело:
select cr.callid, cr.callstart, cr.callend, cr.waittime, cr.origin, cr.origdestination, cr.waitresolution, cr.contacttype, cr.termtype, u.userlastname as assignedagentname, u.userlastname as answeringagentname from callrecord cr left join (agentrecord ar1 left join users u1 on ar.agentkey=u1.userkey) on (cr.callid=ar1.callid and cr.assignedagent=ar1.sequencenumber) left join (agentrecord ar2 left join users u2 on ar2.agentkey=u2.userkey) on (cr.callid=ar2.callid and cr.answeringagent=ar2.sequencenumber);