I coach two niches of clients (main one and additional one) and I would need separate access and different logos for each niche. Is that doable in one coach account or do I need two separate accounts?

It would indeed take another separate system to be branded separately with different/alternate logos.

What some of our members have done in this situation is to brand their TCC website to be of "The Services of {Coach Name}" generic and then include your various logos as graphics on different pages, or in the sidebar content. In essence, you would be creating a third "umbrella" brand and then have your coaching business and coaching services for the organization under the umbrella.

To see what we mean by an 'umbrella' brand, go to www.michaelport.com. Michaelport.com is the umbrella, branded with Michael and his photo. Then, within that site he has several other brands, each with their own logo (and actually each with their own website).