There is a finite amount of time any audience can spend on online communities.
Some people will spend more than others, but no-one can participate in 10 online communities at any one time, most people don’t actively participate in 3.
It makes sense then to partner with other organizations in your sector to create an online community. It opens up your community to a broader audience, lets you double the resources you can put into the community and increases your community’s chance of success.
Even better, every partnership you cultivate removes one competitor for your audience’s attention. Don’t enter into a zero sum game. Realize that those competing for the same audience can be useful allies.