Verifying ability stack

So I have a lvl 2 Lightfoot Halfing Ranger. With lvl 2 I acquired hunter's mark and the two weapon fighting style. 

With the text of hunter's mark stating 'deal an extra 1d6 damage to the target whenever you hit it with a weapon attack' 

In addition, two weapon fighting style adds modifier to damage of 2nd attack. 

So turn scenario, use bonus action spell to hunter's mark the creature then move to close distance then use two shorts words (1d6+3(dex)) per sword to attack. 

Now reading the text, I'm understanding a potential of 15dmg per sword attack with hunter's mark and two weapon fighting style applying affects for a total of 30.

Is this correct?

Not quite. Two-weapon fighting and hunter's mark both take your bonus action, so you can do only one of them in a single turn.

So how about the subsequent turn while the mark is still active? 🤔 

On subsequent turns, you can indeed make both of those attacks.

For two-weapon fighting, the additional attack is as a Bonus Action, so for the first turn, you already used your Bonus Action to cast Hunter's Mark, leaving you with the one attack.

1d6 (Shortsword) + 1d6 (Hunter's Mark) +3 (Ability modifier) = 15 max, or 10 average

On subsequent turns, you can use your Bonus Action to make a 2nd attack, which you add your ability modifier to (both attack and damage) due to the fighting style, Two Weapon Fighting.

2x (2d6+3) = 30 max, or 20 average.