If you want to set multiple Arnold parameters in a User Options string, you can use any whitespace (spaces, tabs, newlines eg \n) that you would use in an actual ASS file.

For example, this:

enable_procedural_cache off error_color_bad_pixel 1 .25 1 declare test constant FLOAT test 2

would add these lines to your options node:

options
{
 error_color_bad_pixel 1 0.25 1

 enable_procedural_cache off

 declare test constant FLOAT
 test 2

Note: Plugins like MtoA and SItoA use AiNodeSetAttributes to set the user options.

The User Options string is a way to set parameters that are not exposed by the plugins. MtoA provides a User Options string for the render options and for shapes. SItoA also has User Options strings for render options and shapes.