Transport Dumper used in exchange 2007

For the Transport Dumpster, must I configured on both CAS Server individually?

To address your concerns one by one:

1. You just need to configure the Transport Dumpster on one of the HUB
servers. Actually it is an universal setting for all storage groups rather
than a server based setting.

For your reference:

How to Configure the Transport Dumpster
http://technet.microsoft.com/en-us/library/aa997963(EXCHG.80).aspx







It is not necessary to increase the transport
dumpster over 18MB. Generally, in a CCR environment, there are only 1-3
transition logs can be delayed. 18MB is that we tested the best results,
so, we set 18MB by default.

The recommended size is 1.5 times the maximum message size that can be
sent. For example, if the maximum size for messages is 10 megabytes (MB),
you should configure the MaxDumpsterSizePerStorageGroup parameter with a
value of 15 MB. If you configure the max send/receive message size to 20MB,
you can set the MaxDumpsterSizePerStorageGroup to 30MB. And as you said
20GB, it is too wasted.