Definition
Double VPN, also called multi-hop or VPN cascading, sends your traffic through two (or sometimes more) VPN servers in succession. Your computer encrypts the traffic for the second server, then encrypts that envelope again for the first server. The first server can decrypt only the outer layer, so it knows your real IP but not your destination. The second server can decrypt the inner layer, so it knows your destination but not your real IP. No single server has the full picture.
When It Helps
A double VPN raises the bar against three threats: a malicious or compromised first-hop server, traffic correlation attacks at a single VPN exit point, and lawful demands directed at a single jurisdiction. By placing the two hops in different countries — say, Switzerland and Iceland — a determined adversary would need to compromise both providers in both jurisdictions to fully de-anonymize the user.
When It Doesn't
A double VPN doubles the encryption overhead and roughly halves throughput. For most users — those whose threat model is 'my ISP is selling my browsing data' or 'I want to watch a different country's Netflix' — the cost is not worth the benefit. For high-risk users like investigative journalists, dissidents, and whistleblowers, multi-hop or Tor over VPN may be appropriate, but no purely network-layer solution defeats sophisticated traffic-correlation by an adversary observing both endpoints.