Two lines under e.g. Union symbol

Question

How does one pile two lines under a union or intersection symbol in display mode? For example

$\bigcup_{a,b \in \mathbb{R}; a+b=x} ... $

displays everything in one line, as it should, but I would like to put those two statements one above the other. I'm sure it's been asked before and I apologise, but I just don't know what to look for.

Answer 1

I have taken like example this old question of @egreg TeX question about double underlining in math mode and after I have added some other examples...

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\def\doubleunderline#1{\underline{\underline{#1}}}
\begin{document}
\[ 
\doubleunderline{\bigcup}_{a,b \in \mathbb{R}; a+b=x} \ldots
\]
\[
\underset{=a,b \in \mathbb{R}; a+b=x}{\bigcup} \ldots
\]
[\bigcup_{\substack{=a,b \in, \mathbb{R}\ a+b=x}} \ldots
]
\end{document}