There’s no reason you can’t have multiple websites. Several of us do it for a variety of reasons:
I’ve been running versions of both for many years and they each have their pros and cons. In terms of IndieWeb support they’re both very solid. Why not try them both for a bit and see which appeals to you more? Depending on your skill level and what you’re looking for in your site you may find one easier to run and maintain than another.
Personally I’ve used WithKnown (I’ve used it for multiple sites since it started) in a more “set it and forget it” mode where I just post content there and worry less about maintenance or tinkering around. On my WordPress site I tend to do a lot more tinkering and playing around, particularly because there is a much larger number of plugins available to utilize without writing any of my own code. Lately I am kind of itching to play around with Drupal again now that it has a pretty solid looking IndieWeb module (aka plugin).