I don't know what you mean by only one field should be selected and the other reset.

Do you mean, if 'account' has a value then 'lead' must be null? And when 'lead' has a value then 'account' must be null?

An example of that is within the communication screen. You can only link a communication record to either a case or to an opportunity because the fields are linked.

You will want to look at these Search Select Advanced fields interact.

See: help.sagecrm.com/.../FC_ChangeDBFieldProps.htm