If a Union Combos is triggered while an Elemental Chain is active or vice versa, the result is a Fusion Combo which merges both bonus damage dices with the attack, increasing the duration of the Elemental Combo by 2 rounds.
Fusion Combos Example*
If Matthew applies an 
Break Inflicting Art onto an Enemy with a Tier 1 “
Fire - Heat” (Referenced in Elemental Combos) already applied;
- Apply On-Hit bonus damage from Union Combos with the appropriate Debuff Stage Chart (I.E. Break,
Topple, 
Launch, 
Smash, & 
Burst). - Reapply the Bonus Damage Dice from Elemental Chain at the appropriate Tier as shown in the Chart.