Your suggested CSS override is the absolute minimum it seems you can "get away" with to get the effect you want, but as you pointed out, it'll affect all other places you want to use the default grid behavior. If jQuery Mobile loads another page into the existing DOM, it won't reload the CSS and you'll be stuck with the widths you've overridden in the other page.
You might be better off making your CSS more specific, for instance add more classes to the div's you want to tweak - say "main" for the wide column and "side" for the narrow one. Then protect yourself against nested grids by using CSS child selectors and the additional classes - that should target precisely the columns you want to tweak and no others:
.ui-grid-a > .ui-block-a.main { width: 65% }
.ui-grid-a > .ui-block-b.side { width: 35%; }
Seems to work like a charm.