That's a really good question! Last year, there was a short story anthology featuring several agented and published writers, and I wondered fleetingly how their agents split the commission. I don't believe each agent would have taken the usual 15%, because there were so many authors involved, if every agent takes 15%, there would be none left for the various authors!
*buys a hot dog and settles down in the corner to watch*
The agents don't split
anything. Agents don't receive commission like that. The writer is the one the publisher pays, not the agent. The publisher sends each agent her
writer's share of the money. She takes out her fifteen percent, and sends the rest on to the writer.
Agents get fifteen percent of what a writer makes, not fifteen percent of what the anthology makes.
Look at it this way. Every fiction magazine is essentially an anthology. If a magazine buy ten stories from ten writers who each have an agent, each agent gets fifteen percent of what her writer was paid for the story.
If the magazine pays a dime per word, and writer one sells a 1,000 word story, he get a hundred bucks, and his agent gets fifteen bucks, taken out of his hundred. If another writer sells a 10,000 word story, he gets a thousand bucks, and his agent gets a hundred and fifty bucks, taken out of his thousand.
An anthology works the same way. The
writer is paid, not the agent. The publisher pays the writer, and the writer pays his own agent.
But the first step would be to talk to your respective agents. Hopefully, they respect each other, and it will be worked out. And then I'd assume each agent would vet the contract and get their commission from their client's share of the advance and royalties. 
