They’re not joining forces, rather, they will both coincidentally be launching their own respective iPad manga service later this year to early next year.
With the wake of their release of a digital-magazine for Yen Plus, Yen Press is pursuing digitized versions of their original licensed works and Korean titles. Japanese publishers are just getting into digital manga distribution these past 6-8 months hence their late start and lack of upcoming titles in Western shores. Yen Press will be selling on a per volume basis at $8.99 USD each.
Dark Horse, in the meantime, has not specified how much they will charge per title or manga chapter but for regular comics, they will be releasing per issue at $1.49 USD each. They will also be experimenting with bundles and free one chapter previews. Most interestingly, they will be synchronizing physical releases with digital releases early next year. Dark Horse has not mentioned what manga titles they will be releasing in digital form but with the new coming coming fast, news will most likely come any day now.