Well the Orlais chantry condemns all mages for blights whereas tevinter condemns old gods only because some Magisters that followed the gods blindly don't count for all mages.
Also the divine in Tevinter is a mage and a very strong one at that.
WoT says they both have the same Chant of Light. That is where the comdeming whomever comes from. If they both recite the exact same chants, they are both teaching largely the same thing. Imperial Chantry has some additions placing Hessarian as the most important of Andraste's disciples and that Andraste was just a prophet and all worship is centered on the maker not Andraste. It doesn't mention anything about blaming the old gods and not magic.
Here is the wiki:
The principal teaching of the Imperial Chantry (and the one which led to the schism with the Andrastian Chantry), is their interpretation of the words, "magic exists to serve man, and never to rule over him." The magisters interpret this to mean that magic must serve the greater good, which they claim to be accomplished through Magi rule. The Tevinter priests altered the Chant to reflect that magic must be used in the service of all mankind for their prosperity and condemned the Chantry in Orlais. This allowed the magisters to reclaim their ancient authority and once again rule the Imperium. Meanwhile, the Chantry in Orlais remains firm in its belief that Magi are untrustworthy and should be controlled. Both Chantries condemn each other about their views on magic.
While the Chantry of Orlais believes that Andraste was divine, and was taken up to stand beside the Maker when she died, the Imperial Chantry maintains she was just a mortal prophet with considerable magical talent, even if she is a symbol of hope. While the Imperial Chantry still respects and honors Andraste considerably, they forbid the worship of her that is practiced in the Orlesian Chantry and instead focus only on worshiping the Maker.