RG Live WotLK 3.3.5a

If 2 Priests cast Hymn of Hope (https://db.rising-gods.de/?spell=64901) at the same time, the buff that increases the mana (https://db.rising-gods.de/?spell=64904) should stack additively, resulting in a 40% mana increase for the target.

http://www.worldoflogs.com/reports/a28vcl4cx8qhioxm/xe/?s=11224&e=11540&x=spell+%3D+"Hymn+of+Hope"%0D%0AAND+targetName+%3D+"Hibyez"

[21:25:11.589] Neyor casts Hymn of Hope
[21:25:13.334] Hibyez gains 552 mana from Neyor's Hymn of Hope
[21:25:13.334] Hibyez gains Hymn of Hope from Neyor

First, the Priest called Neyor casts Hymn. The Shaman Hibyez receives the first tick of Hymn, gains 552 mana from it, and has his mana pool increased 20% after the tick.

[21:25:16.550] Hibyez gains 663 mana from Neyor's Hymn of Hope
[21:25:16.550] Hibyez's Hymn of Hope is refreshed by Neyor
[21:25:18.154] Hibyez gains 663 mana from Neyor's Hymn of Hope
[21:25:18.154] Hibyez's Hymn of Hope is refreshed by Neyor

Since the Shaman now has 20% more mana, the next 2 Hymns restore 20% more mana as a result.

[21:25:18.421] Phlaryu casts Hymn of Hope
[21:25:20.166] Hibyez gains 663 mana from Phlaryu's Hymn of Hope
[21:25:20.166] Hibyez gains Hymn of Hope from Phlaryu

A second Priest, Phlaryu, now casts his Hymn of Hope. Phlaryu gains 663 mana again, but now gains a second Hymn of Hope buff.

[21:25:21.726] Hibyez gains 773 mana from Phlaryu's Hymn of Hope
[21:25:21.726] Hibyez's Hymn of Hope is refreshed by Phlaryu
[21:25:23.412] Hibyez gains 773 mana from Phlaryu's Hymn of Hope
[21:25:23.412] Hibyez's Hymn of Hope is refreshed by Phlaryu
[21:25:24.956] Hibyez gains 773 mana from Phlaryu's Hymn of Hope
[21:25:24.956] Hibyez's Hymn of Hope is refreshed by Phlaryu

The 2 Hymn of Hope stacks increase his mana by 40%. We can deduce that they interact additively because the mana gain going from 552 -> 663 (111 mana gained) is almost equivalent to the mana gained going from 663 -> 773 (110 mana gained).

[21:25:26.204] Neyor's Hymn of Hope fades from Hibyez
[21:25:33.070] Phlaryu's Hymn of Hope fades from Hibyez

Later, both the Hymn buffs are seen fading separately.

---

Can also be confirmed with paladin mana gains:

http://www.worldoflogs.com/reports/a28vcl4cx8qhioxm/xe/?s=11224&e=11540&x=spell+%3D+"Hymn+of+Hope"

[21:25:13.334] Holypalaswe gains 1328 mana from Neyor's Hymn of Hope
[21:25:13.334] Holypalaswe gains Hymn of Hope from Neyor

[21:25:16.550] Holypalaswe gains 1594 mana from Neyor's Hymn of Hope
[21:25:16.550] Holypalaswe's Hymn of Hope is refreshed by Neyor

[21:25:23.412] Holypalaswe gains 1593 mana from Phlaryu's Hymn of Hope
[21:25:23.412] Holypalaswe gains Hymn of Hope from Phlaryu (the second Priest)

[21:25:24.956] Holypalaswe gains 1859 mana from Phlaryu's Hymn of Hope
[21:25:24.956] Holypalaswe's Hymn of Hope is refreshed by Phlaryu

1328*1.2 = 1593.6
1328*1.4 = 1859.2