Short answer is no. If you worked on a piece with other artists, you're encouraged to credit your collaborators in the project description and link to them.
Yes, and this is perfectly fine. In our experience it is extremely rare that artists who worked on the same project post exactly the same image. As ArtStation is a portfolio builder, each artist is entirely in control of what they post. What tends to happen more often is that each artist will present the work they did from their point of view - for example, even though a modeler and lighting artist worked on the same piece, the artwork they post is entirely different as they focus on different parts of the work.
Because artists won't be able to agree. Each artist wants and needs to have complete control over his/her portfolio. They may decide to order the works differently, have different images and texts, put the artworks in different albums, etc. It is impossible to have shared artworks that appear in each other's portfolios because of this.