Ornais died after falling at the fourth fence, while Dooneys Gate died from injuries sustained when he fell at the infamous Bechers Brook.
The league, which wants improvements to racecourses rather than an outright ban on jumping, said Aintree bosses and sponsors "should be ashamed".
In a statement, the league said: "It's not a question of if horses died at the Grand National, it's how many.
"It is ritualised animal cruelty and yet the racing industry stands by and watch, while John Smith's put their profits into sponsoring the cruelty. Everyone involved should be thoroughly ashamed."
Eight celebrities had called for a boycott of the Grand National in a letter which appeared in the Guardian newspaper before the race.
Brian May, Alesha Dixon, Richard Adams, Gemma Atkinson, Emma Milne, Sue Cook, Bill Oddie and Mark Radcliffe lent their support to a campaign by the league for major welfare improvements at Aintree.
"Fifteen horses have died over the last 15 years," said league chief executive Douglas Batchelor before the weekend race.
"If human casualties reached similar numbers there'd be outcry. But when, on average, one horse dies at every Grand National, it calls into question our claim to be a nation of animal lovers if we do nothing about it."
The celebrities' letter branded the event racing's national day of shame, suggesting horses would definitely suffer, if not die.
"It is high time changes were implemented to cut the odds on the number of deaths and to make the Grand National a safer, less cruel event."
Polling by the league in 2010 showed that 65 per cent of respondents thought the course should be changed to lessen the risk.
The celebrities wrote: "We support the league's campaign to boycott the National until improvements to the course are made. We will not be putting a penny on the race and hope the public will join us in forfeiting what may seem like a harmless flutter, or an innocent office sweepstake, in favour of a safer, improved Grand National."