RG Live WotLK 3.3.5a

If multiple Priests cast Divine Hymn (https://db.rising-gods.de/?spell=64843), the 10% healing increase should stack. Right now they just refresh each other.

http://www.worldoflogs.com/reports/7hldmw8k8do00v1g/xe/?s=10737&e=11035&x=spell+%3D+"Divine+Hymn"+and+targetName+%3D+"Kuznam"%0D%0AOR+spell+%3D+"Guardian+Spirit"+and+targetName+%3D+"Kuznam"%0D%0AOR+spell+%3D+"Judgement+of+Light"+and+targetName+%3D+"Kuznam"

[21:12:09.751] Kordellia Judgement of Light Kuznam +0 (O: 1126)
[21:12:11.360] Equity Divine Hymn Kuznam +9872
[21:12:11.360] Kuznam gains Divine Hymn from Equity
[21:12:12.901] Kordellia Judgement of Light Kuznam +1238
[21:12:14.169] Neyor Divine Hymn Kuznam +16584
[21:12:14.169] Kuznam gains Divine Hymn from Neyor
[21:12:15.802] Kordellia Judgement of Light Kuznam +0 (O: 1361)

The JoL goes from 1126 to 1238 upon the first Divine Hymn buff, which is a 10% increase. Then on the 2nd Divine Hymn buff, it goes from 1238 to 1361, which is basically another multiplicative 10%.

http://www.worldoflogs.com/reports/0w1utsdyg1suwv3a/xe/?x=spell+%3D+"Divine+Hymn"%0D%0AAND+targetName+%3D+"Plazma"

[19:20:07.913] Neyor Divine Hymn Plazma +7232 (O: 8389)
[19:20:07.913] Plazma gains Divine Hymn from Neyor
[19:20:11.595] Phlaryu Divine Hymn Plazma +2717 (O: 10889)
[19:20:11.595] Plazma gains Divine Hymn from Phlaryu
[19:20:15.948] Neyor's Divine Hymn fades from Plazma
[19:20:19.549] Phlaryu's Divine Hymn fades from Plazma

http://www.worldoflogs.com/reports/rt-95sc2e364osaot3k/xe/?s=2880&e=3167&x=spell+%3D+"Divine+Hymn"%0D%0AAND+targetName+%3D+"Valithria+Dreamwalker"%0D%0AOR+spell+%3D+"Regrowth"+AND+targetName+%3D+"Valithria+Dreamwalker"

[19:26:06.540] Deuskim Divine Hymn Valithria Dreamwalker +8098
[19:26:06.540] Valithria Dreamwalker gains Divine Hymn from Deuskim
[19:26:10.220] Alsyea Divine Hymn Valithria Dreamwalker +9521
[19:26:10.220] Valithria Dreamwalker gains Divine Hymn from Alsyea
[19:26:10.547] Fokuhila Regrowth Valithria Dreamwalker +19228
[19:26:10.547] Valithria Dreamwalker gains Regrowth from Fokuhila

So far 2 different Priests applied Divine Hymn, about 4 seconds apart, and a Rdruid applies the Regrowth HoT.

Then:
[19:26:13.590] Fokuhila Regrowth Valithria Dreamwalker +5558
[19:26:16.654] Fokuhila Regrowth Valithria Dreamwalker +5558
[19:26:19.048] Deuskim's Divine Hymn fades from Valithria Dreamwalker
[19:26:19.589] Fokuhila Regrowth Valithria Dreamwalker +5053
[19:26:22.577] Fokuhila Regrowth Valithria Dreamwalker +5053
[19:26:22.981] Alsyea's Divine Hymn fades from Valithria Dreamwalker
[19:26:25.616] Fokuhila Regrowth Valithria Dreamwalker +4593
[19:26:28.586] Fokuhila Regrowth Valithria Dreamwalker +4593

We see the Regrowth HoT lose healing as each Divine Hymn buff fades. When the first Divine Hymn buff fades, it goes from 5558/1.1 = 5053. When the second Divine Hymn buff fades, we see 5053/1.1 = 4593