Splitting "Capital Partnerships" (CP) and "Income Stream Investors" (ISI) is an extremely interesting idea for a co-op.  It very neatly divides those who are willing to put their bucks, pounds, and euros behind their beliefs - or mouths :-) - and are willing to take the long-term view or prefer capital accumulation from those who want, or need, a stream of payments or benefits from that capital.  The fights I've witnessed or observed have mostly been from those two camps which, in a sense, are antagonistic in the short term however much they unite in wishing to see the success of the co-op.    

IMHO in order to be consistent with Cooperative principles, it would not matter how many Units you have, you only get one vote.

I agree.  If a person wants to purchase more Units that's their relationship with the operational side of the co-op.  Management and Control issues should be decided by the members and workers - if the latter aren't also the former - based on the Industrial Democratic ideal.  Besides, those more financially invested GET a payback for their investment; asking for more is asking for double compensation, IMO.  

... there are no "dividends" per se ...

haven't been able to think of a better word.  I'd like to have something else to use -- got any lying around?  :-)