Onecolumn command in twocolumn article

Question

I'm making a class (.cls) for CVs, and I'm loading the [a4paper,10pt,twocolumn]{article} class. However, I would like a sort of "heading" section which should only have one column. It's going to look something like this:

I want to define a new environment fullwidth to do this. I have tried:

\newenvironment{fullwidth}[1]{%
  \onecolumn}{\twocolumn}

\RequirePackage{multicol}
\newenvironment{fullwidth}[1]{%
  \begin{multicols}{1}{\endmulticols}

but none of these works. When using onecolumn anything put in the environment does not show up, and multicols just switch to two columns because the multicol package "thinks it's a bad idea to have one column here". Any tips?

The document should look something like (where myclass is the document class I'm writing):

\documentclass{myclass}
\usepackage[utf8]{inputenc}
\begin{document}

\begin{fullwidth}
    A lot of text and the header, printed by the command \makecvtitle .
\end{fullwidth}

Rest of CV in twocolumn
\end{document}