What is the use of cross signing certificates in X.509?

It's about expanding trust, yes. If you trust both CA1 and CA2, and a cert is signed by both, you've got a very high level of trust because two seaparate entities that you trust have verified the cert.

It has the added bonus of increasing the ease of verification of trust, such as situations where you've got clients that trust CA1 or CA2 (but not both). In such a case, you can cross-sign a cert to be trusted by both. This allows more clients to verify trust without having to distribute separate certs for different CAs.

Another bonus is in situations where a CA's private key is leaked. Let's say CA1's key leaks and your cert is signed by CA1 and CA2. In the wake of the leak, CA1 issues a revokation for its public key and you can no longer trust anything issued by CA1. However, since your cert is cross-signed to CA2 as well, any client that trusts CA2 can still maintain a level of trust in your cert.

The X.509 specs only support one signature. From the RFC concerning them:

   Certificate  ::=  SEQUENCE  {
        tbsCertificate       TBSCertificate,
        signatureAlgorithm   AlgorithmIdentifier,
        signatureValue       BIT STRING  }

To support multiple signatureValue's you'd have to do something like "signatureValue SEQUENCE OF BIT STRING" and make a few other changes too.

If you have an X.509 cert that's been signed by multiple CA's I'd love to see it and do a little asn1parse on it!