There are both advantages and disadvantages having two firewalls. While firewalls are not commonly exploited, they are prone to denial of service attacks.
In a topology with a single firewall serving both internal and external users (LAN and WAN), it acts as a shared resource for these two zones. Due to limited computing power, a denial of service attack on the firewalls from WAN can disrupt services on the LAN.
In a topology with two firewalls, you protect internal services on the LAN from denial of service attacks on the perimeter firewall.
Of course, having two firewalls will also increase administrative complexity - you need to maintain two different firewall policies + backup and patching.
Some administrators prefer to only filter ingress traffic - this simplifies the firewall policy. The other practice is to maintain two seperate rulesets with both outbound and inbound filtering. If you need an opening from LAN to WAN, you will have to implement the rule on both firewalls. The rationale behind this is that a single error will not expose the whole network, only the parts the firewall is serving directly. The same error has to be done twice.
The main disadvantage is cost and maintenance, but in my opinion the advantages outweighs these.