Can I open multiple concurrent Excel Workbooks (using Excel
Calculation Services, ex:
http://devServer/_vti_bin/ExcelService.asmx) and keep these sessions
open for 3-minutes?

We have a requirement that needs to maintain 20 concurrent sessions
to the Excel Calculation Services for at least 3 minutes each
(preferable 10 minutes).

Is there any special configuration for Excel Services to support this?

For testing I wrote a small program (windows form) to open an Excel
workbook on a MOSS trusted location. This program works as long as I
execute one instance of it at a time.

To simulate a user connection and keep the session open, my program
stays in a loop for 3 minutes while repeatedly calculating and
obtaining results from the Excel workbook.

This program works fine if I execute only one instance (one thread,
i.e. one user).

If I execute more than one instance of the program, it will run for
awhile, access the Excel workbook hundred times and then it gets one
of the following errors:

1) "Error: An operation is already being processed in this session.
Only one operation can be processed in a session at a time".
2) "Your session has timed out because of inactivity."
3) "Unable to connect to the remote server"

I've modified
1) the Excel Calculation Server web.config file to include:
<system.net>
<connectionManagement>
<add address="*" maxconnection="10" />
</connectionManagement>
</system.net>
2) the IIS connection timeout for the Office Server Web Services to
be 1200 seconds

I got the similar results from either a single server (1 CPU & 2GB
Memory) MOSS environment or a 3-server (1 WFE + 1 App + 1 SQL )
environment – the WFE server has 1 CPU and 2GB memory; the
Application server has 1 CPU and 4 GB memory; the SQL server has 1
CPU and 4GB memory. I configured Shared Services to run on the
Application server.