Bottom line answer - if it is your event, sponsored by your company, you can do whatever you want to, and benefit whomever you wish. Neither organization should be telling you what to do. If you do an OCF event that we are helping you organize and pay for, then we expect you to work with us exclusively, I am sure that SPOHNC feels the same in that regard.
If you would like to call me next week I would be happy to talk to you about how we do our events, and how we work with those that help us with them. As to working with SPOHNC, for over a decade we have been co-sponsors of April's awareness month, each doing our own things, but working toward mutual goals. We share some advisory board members that are on both non-profit's boards. When Chattanooga SPOHNC support group leader Jeanna wanted to put on an event, she did it to support OCF, realizing that when it came to funding, we have a fraction of the assets of SPOHNC. The leadership of SPOHNC had no issue with it, and she still is their local facilitator. And as far as communication between the two, I talked just this morning with the executive director (my counterpart) @ SPOHNC about common issues we have.
But this is YOUR event, and you should do what you think best. Call me and I will give you details if you want to know more. There are distinct differences between the two organizations, and I would welcome the opportunity to discuss with you what they are.