If you've already read "breaking the 9-argument limit" you can probably guess the solution to this problem: command relaying.
LaTeX allows commands with a single optional argument thus:
\newcommand{\blah}[1][Default]{...}
You may legally call such a command either with its optional argument
present, as
\blah[nonDefault] or as \blah; in the latter case, the
code of \blah will have an argument of Default.
To define a command with two optional arguments, we use the relaying technique, as follows:
\newcommand{\blah}[1][Default1]{%
\def\ArgI{{#1}}%
\BlahRelay
}
\newcommand\BlahRelay[1][Default2]{%
% the first optional argument is now in
% \ArgI
% the second is in #1
...%
}
Of course, \BlahRelay may have as many mandatory arguments as are
allowed, after allowance for the one taken up with its own
optional argument - that is, 8.
A command with two optional arguments strains the limit of what's sensible: obviously you can extend the technique to provide as many optional arguments as your fevered imagination can summon. However, see the comments on the use of the keyval package in "breaking the 9-argument limit", which offer an alternative way forward.