I did this with my husband. I called customer service and told them I wanted to transfer 30,000 Marriot rewards to my spouse so that he could transfer that to 10,0000 SW points, which got him to Companion Pass Status.
They mailed me the form, we signed it, and sent it in. Once they received the form it took 48 hours and then the points were in the SW account. From phone call to points received was a one week process. It would be faster if you used a fax machine.
Obviously the conversion rate is not that great, but was worth it for CP.
Not sure if this is only available to legal spouse or to any two Marriott reward members.
Good Luck!