How does spell tinker arcanist exploit work with extend spell?

Question

I am playing a arcanist and I am considering taking the exploit spell tinkerer.

I am only interested in the increase duration effect, not the reduced duration for the purposes of this question.

I need to know how it works with the metamagic feat extend spell?

The way I see it there are 3 possibilities

1. They do not stack, any given spell can only be increased by one of the effects and not both.
2. They stack but are separate. The spell's duration becomes 250%.
3. They stack together. The spells duration becomes 300%.

Answer 1

Any remaining duration will stack together

As per the text of Spell Tinkerer:

The arcanist can alter an existing spell effect by expending 1 point from her arcane reservoir. To use this ability, she must be adjacent to the spell effect (or the effect’s target) and be aware of the effect ... This ability cannot be used on a given spell effect more than once.

This means that while you're not able to modify the spell as you are casting it, you may use a melee touch attack and the spell tinkerer feature to again extend the spell.

While Spell Tinkerer does call out that it will not effect a spell more than once, it doesn't refer to any other effect which can be used to extend duration.

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Assuming a spell with a standard duration of 10 Rounds:

• Round 1: Cast the spell using Extend Spell (20 Rounds Remaining)
• Round 2: Spell looses 1 round. Spell Tinkerer will add 50%(9 Rounds), total 28 rounds remaining).