I learn to use Eleventy recently and i successfully
combining 2 template. I don't know if someone already figure this out but it's kind of fun to do it. The attached link is the result of combined 2 template.
I use template from sprucecss-eleventy-documentation-template for documentation and vredeburg for blog or article.
What i do to combine it is simply make on of the template as home and the other as sub pages so its not use subdomain.