The simple checkbox mode allows only one base per line. On top of each of the checkbox grid for each of the apparatus, you will see a highlighted mode change button / link that looks something like this, click on it to allow yourself full control over what is on one line - you can have two or more bases on a line using this mode.

Furthermore, this is the only way to control the sequence of the criteria symbols.

The following is for illustration purposes only:

Click to handle multiple bases in a row, repeating elements and special sequences