\starttext
\placeformula
\startformula
	a⁺ \quad b⁻
\stopformula
\placeformula
\startformula\startalign
	a⁺ \NC≠ \NC b⁻ \NR
	a^+ \NC≠ \NC b^- \NR
\stopalign\stopformula

It should work though $a⁺$ and $b⁻$.
\stoptext