You can change the host header going in and out of the LTM, however you'll still get cert errors unless the cert has both host names in it (The SSL host name check happens before the LTM iRules get involved), it should work fine for HTTP only then.
You could also have a redirect which would send people to the correct URL.
when HTTP_RESPONSE {
HTTP::header replace Location [string map -nocase {xyz.test.com abc.test.com} [HTTP::header value Location]]
}
when HTTP_REQUEST {
HTTP::header replace Host [string map -nocase {abc.test.com xyz.test.com} [HTTP::header value Host]]
}