Does creating a second potion with Sinderion’s Serendipity give me twice the XP

The Smithing perks are a touch misleading.

First, consider this major point:

You don't need any smithing perks to improve any (non-magical) piece of armor. If you have the right material, you can improve it at the appropriate place. This will temper your weapons to (e.g.) (Fine) quality descriptor.

The smithing perks let you be more effective with your tempering at lower smithing skill, so instead of upgrading your gear to (Fine), you can upgrade it to (Superior).

The UESP has compiled this handy table to explain the effects of tempering.

Dwarven Oil and Taproot have the exact same effects: Weakness to Magic, Fortify Illusion, Regenerate Magic, and Restore Magicka.

If you create a potion from these ingredients, it would have all four effects (3 positive, 1 negative). With the Purity perk, the negative effect would not appear. As a potion, you can drink it.

If you create a poison from these ingredients, it would have all four effects (3 positive, 1 negative). With the Purity perk, the 3 positive effects would not appear. As a poison, you can put it on your weapon.