Im having a mail id in google apps for business for one or my domain.since my user accounts has been full (reached 10 users per account).so that i bought yahoo mail service. is it possible to keep both google and yahoo mx in a single domain?
Technically - yes. You can have as many MX records you like.
It will not work in practice though. Mail servers who are trying to send you mail does not know which server holds whose mailbox. In other words, if user joe has his mailbox on google apps and a mail server tries to deliver it to yahoo it will get denied, unless joe also has a mailbox on yahoo.
Stop being a cheapskate and just upgrade your account :-)