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

Yes! It is about expanding trust. If you trust both CA1 and CA2, and a cert is signed by both, you've obtained a very high level of trust because two separate entities that you trust have verified the cert. It has the added advantage of increasing the ease of verification of trusts, 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 revocation 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 certificate.

What’s cross signing?  Cross-signing is simply when multiple valid paths exist between a root certificate and a node certificate. This can be advantageous for a number of reasons. Sometimes, a certificate in the chain expires. Strategically cross-signing intermediate certificates from an older (therefore more likely to be present on the larger subset of devices in the wild) root certificate, “buys some time”.Cross-signature is also useful when CAs need to perform maintenance.