Hi All!
I have found the reason on which two or more forms containing a field captcha math, on one page incorrectly worked.
The reason in that from Prepare Form through session two global variables - a field name captcha and the correct result connected with this name were transferred.
As a result, Prepare Store received a name and result of last opened captcha.
If a name not to transfer through session, and to take from parametres Prepare Store the correct result receives from the necessary global variable.
Here that for this purpose it is necessary to make in a file /plugins/cck_field/captcha_math/captcha_math.php
1. In section onCCK_FieldPrepareForm - Set it is necessary to remove a line 112
$session->set( 'secure_cckaptcha', $name );
2. In section onCCK_FieldPrepareStore - Validate Captcha it is necessary to replace a line 169
$captcha = $session->get( 'secure_cckaptcha' );
For the line
$captcha = $name;
Then two or more forms with captcha math on one page will work correctly provided that for each form the field captcha with the unique name will be created.