Can siblings have joint POA?

Yes absolutely, though I think it is a great mistake. No two people in the entire world agree with how to manage things. There could be constant argument and disagreement. It is by far best to have one sibling best willing and qualified to do this work (and it IS WORK) and record keeping, with the second sibling a second in case first is unable to serve either temporarily or permanently.

Yes, but it takes extraordinary cooperation between 2 people to make it feasible. In my family, it would have been a disaster

Yes, they can, but it isn't a great idea. Both have to agree on things, and a lot of institutions that deal with a person holding POA want to deal with ONE person with that power, not more.

Yes, it all depends on how the assignee wants the attorney to write it up. My husband and his brother are both DPoA for their mom. They get along and so far my MIL hasn't had any issues that could create disagreement. Whoever has more than one PoA needs to give very specific instructions on their care and decisions so that disagreements and misunderstandings can be avoided as much as possible. Even then it isn't guaranteed.