Less and Sass at the same time

I’m migrating an Admin LTE (less) template to Metronic (sass) and doing it gradually I want to keep the current app.less as it is and just add custom.scss to the end up as app-custom.css or alike. The question is how do to it without conflicts?

If I add ember-cli-sass then it will want to do app.scss into app.css while ember-cli-less will want to do the same with app.less and one will override the other. I tried removing both and using ember-cli-css-preprocess and specifying the “app” and “custom.scss” in outputPaths but then the style build process seems to get stuck and never finishes.

Anyone has some idea how to solve this problem?