Brain and Terry worked together on the "California Music" project!
Regarding California Music WHY DO FOOLS FALL IN LOVE, it was more like Terry rescuing a track that Brian had been contracted to produce. You can hear Brian's arrangement and production touches in the basic tracks: sleigh bells, accordion, loopy drum part, bari and tenor pad parts along with his background vocals. It sounds like that's as far as Brian got on the production of the song. I don't think there was much collaboration involved, even if Terry is the lead vocalist on the record.
In the bridge of Why Do Fools Fall In Love, you can hear where Terry takes over the production reins. Also, the last part of the song does not contain much meat. The bg vocals are missing and there's just miscellaneous banter between Terry and the female vocalist over the rhythm track.