As the title suggests. Say an enemy is burning and under the effect of Pain strike.
These are two forms of continuous damage, so my question is if the enemy tries to cast a spell does he make 2 concentration checks one with a DC = 10+.5(burn damage)+SL and then one with DC 10+.5(pain strike damage) + SL, or 1 concetration check with DC = 10+.5(burn damage)+.5(pain strike damage)+spell level?
Concentration
If you are taking continuous damage, such as from an acid arrow or by standing in a lake of lava, half the damage is considered to take place while you are casting a spell. You must make a concentration check with a DC equal to 10 + 1/2 the damage that the continuous source last dealt + the level of the spell you’re casting. If the last damage dealt was the last damage that the effect could deal, then the damage is over and does not distract you.
From the bolded section I'd think you "Are considered" to be taking damage from both sources while trying to cast the spell and the damage from each should add the DC.
Therefore the DC is: 10 + .5 (pain strike damage) + .5 (burn damage) +SL