Now I have a macro with two arguments for parameters in Context style:

\def\starttablex{\dodoubleargument\dostarttablex}

\def\dostarttablex[#1][#2]{%

Now for input: \starttable[][] & ... all is well,

but for input: \starttable[]   & ... the ampersand appears to be eaten up by the double argument expansion.

How can I prevent that, letting & end the parameter scanning and using it as the first input?