Tom, let me recap a bit. The simplest solution for you would be achieved if you could definitively rule out the possibility that the partnership is part of an Affiliated Service Group (ASG) with you and your co-owner's S-Corp. If so, then it is perfectly permissible for your S-Corp to sponsor two separate 401(k) plans, one for each of you.
However, nothing you have written allows a definitive conclusion regarding the existence or lack thereof of an ASG. If you can state the nature of the partnership (that is, what does the partnership do to make money) and the nature of what you do to earn the approximate $20,000 that does not come from the partnership, it just MIGHT be possible to rule out the existence of an ASG.
Fair warning: it is far from certain that even if you provide this information a definitive conclusion can be reached. It is far more likely that it will just lead to more questions.
If you are trying to make a determination before the end of the year (for whatever reason) I urge you to abandon these forums and hire somebody (not me, I'm far too busy between now and the end of the year).