RG Live WotLK 3.3.5a

• Skillt man ausschließlich Heilige Konzentration, wird der Manareg erhöht, wenn kein Zauber gewirkt wird
• Skillt man Heilige Konzentration und Meditation, wird bei einem Procc der Manareg sowohl während des Zauberwirkens als auch außerhalb davon erhöht

https://wow.gamepedia.com/Holy_Concentration_(priest_talent)

Cataclysm Patch 4.0.1 (2010-10-12): Re-designed: In-combat mana regeneration from Spirit is increased by an additional 10/20%.
Wrath of the Lich King Patch 3.1.0 (2009-04-14): Re-designed: Mana regeneration is increased by 16/32/50% for 8 sec after you critically heal with Flash Heal, Greater Heal, Binding Heal or Renew.

https://www.mmo-champion.com/threads/638210-3-1-Meditation-and-Holy-Concentration

The new Holy Concentration works on spirit-based mana regen, and basically adds 50% to your spirit while the buff is up (for mana regen purposes).
It does not work on gains from pure MP5 gear, nor other incomes like replenishment, shadowfiend, etc.

If you do not have meditation, spirit will give you absolutely nothing inside the FSR. So, yes, these will stack. But no, you will not have 100% FSR regen.

Assume you have 600 spiritbased manaregen OOFSR, and 300 FSR (note, this is not the numbers on your character sheet, this is the benefit from spirit).
Holy Concentration will add 50% to this regen. That means 900 manaregen OOFSR and 450 FSR while the buff is up. But not 900 FSR.

You can argue that if you look at the FSR numbers alone, holy concentration is worth exactly half the value of Meditation. In this case, meditation is worth 300 MP5, and holy concentration is worth 150 MP5. Assuming the buff has a 100% uptime. I'm not sure if it will.