Our network currently has two firewall systems: Fortinet's FortiGate and Cisco's PIX Firewall. To reduce processing delays, I suggested having one firewall, instead of both devices. What are the security advantages of having multiple firewalls from different vendors?
You raise a few interesting issues in your question. First, your underlying assumption is that the two firewalls are introducing a noticeable latency into your network. Generally speaking, when using modern firewalls that have capacity suitable for a modern enterprise network, there shouldn't be any significant delay, even when chaining two firewalls together. If you're experiencing a delay, it's probably a sign that one or both firewalls can't keep up with bandwidth requirements, hence it may be time to upgrade to a higher-capacity model.
Next, consider the placement of the firewalls. Are they both directly connected to each other with nothing else in between? If that's the case, consider using a different firewall topology that will get the most out of the two firewalls. For more information on arrangements, read my article Placing systems in a firewall topology. If the firewalls aren't directly connected, make sure that they're not already part of a multi-homed topology implementation.
If there's no business case for redundant firewalls, it might be worthwhile to decommission one of them. Rather than emphasizing performance reasons, I'd cite the hassle and management overhead of maintaining two different systems. Rule changes must be made on both devices separately, effectively doubling administration time. If you're interested in achieving redundancy to protect against a hardware failure, however, consider using two of the same device in a high-availability deployment so that if one firewall fails, the other takes over.
