thanks for providing us with additional details.
There's not a direct way to achieve this as the MoSpline only has one width parameter and it's not possible to split this for segments of the input spline.
But if you unfold the width parameter (the small black triangle at the beginning) you are being offered a Formula parameter. By this you can achieve it, even if the way seems a bit twisted.
With the Formula you can do something like this:
if (segment == 0; 0.5; if (segment == 1; 1.0; 0.0))
The syntax is
if (cond; cond is true value; cond is false value). In my example above it will set the radius to 0.5 if segment index is zero and then in the "else branch" there's another nested if clause, setting the radius to 1.0 for segment index equal 1 and for all other segments it sets the radius to 0.0. By nesting more and more if clauses you can basically achieve, what you want. And you could use either Xpresso or a Python Tag to concatenate the formula string as needed.
Not the most straight forward solution. But it should do the trick.
I have moved this thread to Cinema 4D Development category.