fontsunicode-math
I'd like to typeset a document that uses two different math fonts with unicode-math. In the documentation I can't find anything about switching the font mid-document. Just writing
\setmathfont{xits-math.otf} $a$
\setmathfont{lmmath-regular.otf} $a$
does work, but I'm wondering whether this is the correct way to do it when switching between fonts several times in a document.
After defining new math versions with \DeclareMathVersion, you can use \DeclareMathAlphabet, \SetMathAlphabet, \DeclareSymbolFont and \SetSymbolFont to set all math families manually. You must rename the math family names. You must be sure there are no more than 16 math alphabet families (but symbol familis can be more). And there may be encoding problems.
Anyway, although it's quite complex, it is posible:
\documentclass{article}
%%% text fonts
\renewcommand\rmdefault{txr}
\newcommand\gmfamily{\fontfamily{mdugm}\selectfont}
%%% New math versions
\DeclareMathVersion{varnormal}
\DeclareMathVersion{varbold}
\newcommand\txmath{\mathversion{normal}}
\newcommand\txboldmath{\mathversion{bold}}
\newcommand\mdmath{\mathversion{varnormal}}
\newcommand\mdboldmath{\mathversion{varbold}}
%%% Math symbol fonts
%%% some examples only
% Math letters from txfonts and mdugm
\SetSymbolFont{letters}{normal}{OML}{txmi}{m}{it}
\SetSymbolFont{letters}{bold}{OML}{txmi}{bx}{it}
\SetSymbolFont{letters}{varnormal}{OML}{mdugm}{m}{it}
\SetSymbolFont{letters}{varbold}{OML}{mdugm}{b}{it}
% Math operators
\SetSymbolFont{operators}{normal}{OT1}{txr}{m}{n}
\SetSymbolFont{operators}{bold}{OT1}{txr}{bx}{n}
\SetSymbolFont{operators}{varnormal}{OT1}{mdugm}{m}{n}
\SetSymbolFont{operators}{varbold}{OT1}{mdugm}{b}{n}
% Math symbols
\SetSymbolFont{symbols}{normal}{OMS}{txsy}{m}{n}
\SetSymbolFont{symbols}{bold}{OMS}{txsy}{bx}{n}
\SetSymbolFont{symbols}{varnormal}{OMS}{mdugm}{m}{n}
\SetSymbolFont{symbols}{varbold}{OMS}{mdugm}{b}{n}
% Large symbols
\SetSymbolFont{largesymbols}{normal}{OMX}{txex}{m}{n}
\SetSymbolFont{largesymbols}{bold}{OMX}{txex}{bx}{n}
\SetSymbolFont{largesymbols}{varnormal}{OMX}{mdugm}{m}{n}
\SetSymbolFont{largesymbols}{varbold}{OMX}{mdugm}{b}{n}
%%% Math alphabets, at most 16 families
%%% some examples only
\SetMathAlphabet{\mathrm}{normal}{OT1}{txr}{m}{n}
\SetMathAlphabet{\mathrm}{bold}{OT1}{txr}{bx}{n}
\SetMathAlphabet{\mathrm}{varnormal}{OT1}{mdugm}{m}{n}
\SetMathAlphabet{\mathrm}{varbold}{OT1}{mdugm}{b}{n}
\SetMathAlphabet{\mathit}{normal}{OT1}{txr}{m}{it}
\SetMathAlphabet{\mathit}{bold}{OT1}{txr}{bx}{it}
\SetMathAlphabet{\mathit}{varnormal}{OT1}{mdugm}{m}{it}
\SetMathAlphabet{\mathit}{varbold}{OT1}{mdugm}{b}{it}
\begin{document}
This is Times font. $a^2 + b^2 = c^2$. Math font
\[
\sum_i \int_a^b \left( \frac1{K+1} \oplus\alpha_i \right) \,\mathrm{d}x = 0
\]
and bold math font
{\txboldmath\[
\sum_i \int_a^b \left( \frac1{K+1} \oplus\alpha_i \right) \,\mathrm{d}x = 0
\]}
\gmfamily\mdmath
This is Garamond font. $a^2 + b^2 = c^2$. Math font
\[
\sum_i \int_a^b \left( \frac1{K+1} \oplus\alpha_i \right) \,\mathrm{d}x = 0
\]
and bold math font
{\mdboldmath\[
\sum_i \int_a^b \left( \frac1{K+1} \oplus\alpha_i \right) \,\mathrm{d}x = 0
\]}
\end{document}
the release date for Minion Math 1.020 with MATH table support is October 2011 (as I still need to perform some tests before release).
In my tests so far Minion Math works (with fontspec and unicode-math) with LuaLaTeX and XeLaTeX, with slightly better results / less problems in LuaLaTeX.
Optical sizes in math do work (with appropriate options for \setmathfont).
Minion Math does fully support 4 different weights. In the current setup, Bold Math (Regular and Italic) is taken from a bolder weight, so your example will need a few extra commands (\setmathrange) and will not work exactly, but almost "out of the box". The \fontsize commands should work (I will check that).
Best Answer
unicode-math, starting from v0.6, supports LaTeX math versions, which is a more efficient way for switching math fonts mid-document than re-loading the font each time: