This message was deleted.

if they address the same or a similar target audience, go for it! it becomes more than just seo then; it is like piggybacking on their site to extend your reach 🙂

Go for collaboration. Ask yourself "What is in it for them?" Leads? Backlinks? Traffic? And what about the content? Are you going to use the same content (which is not recommended) or a canonical URL? How are you going to introduce the topic? Perhaps do a joint webinar/interview to warm up the reader. It is not scalable but could be worth pursuing if you think about its distribution.