How to get two versions of the same math Unicode symbol from different fonts with XeLaTeX or LuLaTeX?

Question

Let take my favorite symbol „equals by definition“, ≝.

Here's what I tried so far to get a short and a long version of this symbol (yes, I wish to have both in the same document):

\documentclass{article}
\pagestyle{empty}
\usepackage[math-style=ISO]{unicode-math}
\setmainfont[Ligatures=TeX]{TeX Gyre Termes}
\setsansfont{TeX Gyre Heros}[Scale=0.88]%%% Somewhat ok scaling.
\setmonofont{TeX Gyre Cursor}%%% No explicit turning on ligatures for the monospaced font.
\setmathfont[Ligatures=TeX]{TeX Gyre Termes Math}
\setmathfont[Ligatures=TeX,Extension=.otf,range={"2A3E},BoldFont=XITSMath-Bold]{XITSMath-Regular}%%% The fat semicolon comes from XITS.
\newcommand*{\longDefiningEquals}{\mathrel{\text{\setmathfont{latinmodern-math.otf}[Ligatures=TeX,Extension=.otf,range={"225D}]$≝$}}}%%% long equality symbol that is used to define stuff
\begin{document}
\begin{gather*}
  R⨾\ ≝ \{ (,) \mid ∃ \colon (,) ∈  ∧ (,) ∈ \}\\
  R⨾\ \longDefiningEquals\ \{ (,) \mid ∃ \colon (,) ∈  ∧ (,) ∈ \}\\
\end{gather*}
\end{document}

The output (here, from lualatex)

looks o.k., but the input looks cumbersome: to get a single math symbol in an alternative font, we switch to text mode, change the math font, switch to math mode again, and finally issue the symbol itself. Further, in the symbol happens to be needed as a relation, we stuff all this junk into \mathrel{…}. How can be input (in particular, the macro defining the alternative glyph for the symbol) be conceptually or textually simplified (for the purpose of getting an alternative glyph for a symbol already used)?

Answer 1

There is no need to setup again and again a math font only to access a character in a font.

\documentclass{article}
\usepackage[math-style=ISO]{unicode-math}
\setmainfont{TeX Gyre Termes}
\setsansfont{TeX Gyre Heros}[Scale=MatchLowercase]
\setmonofont{TeX Gyre Cursor}[Scale=MatchLowercase]
\setmathfont{TeX Gyre Termes Math}
\setmathfont{XITS Math}[range={"2A3E},BoldFont=XITSMath-Bold]
\newfontface\latinmath{latinmodern-math.otf}

\newcommand*{\longDefiningEqualsB}{%
   \mathrel{{\mbox{\latinmath ≝}}}}
\begin{document}
\begin{gather}
 u≝v \ 
 u\longDefiningEqualsB v \
\end{gather}
\end{document}

Answer 2

I can't see a reason for using both in the same document, but certainly the way is not with \setmathfont, which does an awful lot of things.

Trusting on (but not endorsing) your judgment of having two copies of the same symbol in different styles, your idea of using \text is good, but you just need to choose a text font and nothing prevents you from declaring a math font as a text one. The converse is problematic: in order to be useful for math, an OpenType/TrueType font must have special tables in it.

The appropriate command is \newfontface.

Note that you almost never want Ligatures=TeX, which the default except for monospaced fonts. Stick to one style for the options when loading fonts. The preferred way is options after the font name.

\documentclass{article}
\usepackage[math-style=ISO]{unicode-math}
\setmainfont{TeX Gyre Termes}
\setsansfont{TeX Gyre Heros}[Scale=0.88]
\setmonofont{TeX Gyre Cursor}
\setmathfont{TeX Gyre Termes Math}
\setmathfont{XITSMath-Regular}[
  Extension=.otf,
  range={"2A3E},
  BoldFont=XITSMath-Bold,
]
\newfontface{\latinmodernmath}{latinmodern-math.otf}
\newcommand*{\longDefiningEquals}{\mathrel{\text{\latinmodernmath\symbol{"225D}}}}
\begin{document}
\texttt{``aa''}% see? No ligatures
$A\longDefiningEquals B$
$\scriptstyle A\longDefiningEquals B$ % scales!
\begin{gather}
  R⨾\ ≝ { (,) \mid ∃ \colon (,) ∈  ∧ (,) ∈ }\
  R⨾ \longDefiningEquals { (,) \mid ∃ \colon (,) ∈  ∧ (,) ∈ }\
\end{gather}
\end{document}