Code:
#showtooltip
/cast [@mouseover,help,nodead,mod:shift] [help,nodead,mod:shift] [mod:shift] Flash of Light; [@mouseover,help,nodead] [help,nodead] [] Holy Light
Change “mod:shift” to the modifier key you want to use. If you wanted to cast the other spell on right-click instead of with a modifier key, change it to “btn:2”.