Collaboration and support are the key hallmarks of the indie podcasting community.
Cross promotions, guest appearances and so on are part of this process. As a general rule, people will happily support similar podcasts to their own.
The reasons for this are instructive. Each show appeals to each own niche audience and the voice of the presenter. By interacting with similar type shows we allow listeners a greater choice. The other show’s listeners may just be as interested in your style of show and your listeners may like the cross promoted one. There no real limits other than the 24 hours in a day to the number of podcasts we can subscribe to.