Combo skills are hidden but once someone or your YOME use a certain skills that can be partnered to current 3 skills it will be unlocked. You will hear a sound (flash thingy) when it happens. Don't have to worry if for the first you fail to use the your skill, it will still be unlock. You can check it at Combo skills list.

Name Description Skills needed Video
Blitzspeer A sparkling burst of magical power. Magic damage to single opponent. Lightning(Explorer) + Vertical Edge(Explorer)
Twin Edge Rampage A soaring blade, sweeping sword. Physical damage to single opponent.
Twin Edge Rampage
Dual Arts Magic and swordplay tag-team attack. Physical damage to single opponent.