Tl;dr: Any damage or healing overtime component will always stack, but any constant effect from a buff or debuff will never stack.

Examples:
Heal overtime.
White Mage 1: Regen on Paladin. (+100 HP every 3 seconds)
White Mage 2: Regen on Paladin. (+100 HP every 3 seconds)

Paladin will gain both Regen's (stacking) heal overtime effect every 3 seconds. (+200 HP per 3 seconds)

Damage overtime.
Scholar: Shadow Flare on Training Dummy. (40 damage dealt every 3 seconds)
Summoner: Shadow Flare on Training Dummy. (55 damage dealt every 3 seconds)

Training Dummy will be inflicted 95 damage collectively between the two Shadow Flares every 3 seconds. The Training Dummy is constantly slowed for the duration of Shadow Flare.

BUT!

Only one instance of the slow from Shadow Flare is active. This means the slow does not stack.