[XeTeX] How to mix math fonts?

[Mojca Miklavec]

Maybe an easier question would be the following. To compare text fonts
I could do the following (pseudocode):
    \font\a="Some font:mapping=tex-text"
    \font\b="Some Other Font:mapping=tex-text"

    \def\printline#1{{\a #1}{\b #1}}
    \somecommandtoloopoversomerange{0}{1000}{\printline{\char\loopnumber}}


This command (\a and \b) has very low penalty. My question is: how can
I achieve the same low-level with math? The command
    \setmathfont{Asana Math}
takes a lot of resource and is not suitable for very frequent changes of fonts.

Mojca