Composite Alternative Key in Custom BO

Hi Experts,

I have a requirement to develop a custom bo with 2 fields as composite alternative key. And then create/update data using web service.

[AlternativeKey] element key1:ID;

[AlternativeKey] element key2:NumberValue;

With this code, when i try to insert the data (using SoapUI) by changing one of the key field values,

For example:

case1: key1 = "123", key2 = "321" --> success,

case2: key1 = "123", key2 = "456" --> fails

it gives "Instance with the same key already exists" error, considering both the fields as individual keys.

Is it possible to make 2 fields as composite alternative key ? and is there any other way to do it ?

Thanks & Regards,

Sadhna