Swish⁽¹⁾ lets you drag the divider to resize multiple windows at once. BentoBox⁽²⁾ is inspired by Fancy Zones. And Lasso⁽³⁾ is a grid-based window manager with custom layouts. There's also MacsyZones⁽⁴⁾ that appears to resize multiple adjoining windows but I've never used it (it appears to be open-source with an option to pay to support the author).

⁽¹⁾ https://highlyopinionated.co/swish/

⁽²⁾ https://bentoboxapp.com/

⁽³⁾ https://www.thelasso.app/

⁽⁴⁾ https://macsyzones.com/