Concat AST with CSSTree. Note that CSSTree lost an original formatting on parsing.

concat-ast.js

const csstree = require('css-tree');

const ast1 = csstree.parse('.a { color: red }', {
  filename: './a.css',
  positions: true
});
const ast2 = csstree.parse('.b { color: red }', {
  filename: './b.css',
  positions: true
});

const ast = {
  type: 'StyleSheet',
  children: new csstree.List()
    .appendList(ast1.children)
    .appendList(ast2.children)
};

csstree.generate(ast, { sourceMap: true });
// { css: '.a ...', map: SourceMapGenerator }