Are Undergraduates allowed to collaborate with other students for their dissertation research?
Yes !

Undergraduates are definitely allowed to collaborate with other students (undergraduate/graduate) for their senior thesis/research project/dissertation etc.

Of course, the degree to which they can successfully collaborate depends on many things including their faculty adviser.

For instance, as an undergraduate I collaborated with students from Civil Engineering/Construction Management and used some of the results we obtained in my undergraduate thesis.

Currently, I am a doctoral candidate and I have worked with undergraduates in the past whose thesis has sprouted from the results of our active collaboration in a project.