my question is can we (my brother and I) submit this proposal for the university so we both share the research?
Yes, assuming the university and supervisors don't object.
do we have to provide 2 seperate research thesis?
Yes (in general). You can work together throughout, but, ultimately, both of you need to uniquely identify your individual contributions. I'd suggest that one of you takes the lead on each work package and that work package is considered their individual contribution, whilst the other provides support. Resulting publications will list both of you, but such publications will only appear in one thesis. (There's some flexibility here, since you might both include material from some publication. Your supervisors can help you work that out.)