Are donkeys more likely to have twins?

Yes. And the twins are more likely to survive. (This is also somewhat the case with mares carrying mule foals). There's even been five recorded cases of mares producing twins where one was a mule and the other a horse (because she was thought to be "open" and bred again).

However, although they're more likely to survive, the ratio is still pretty low and twins are still considered undesirable in donkeys. (The chances of both twins surviving and being healthy is about 1 in 1000 in horses and 1 in 100 in longears).

Image source: Adrian Pingstone via Wikimedia Commons.