May sound odd, but we have our normal network setup with one SEPM and a lot of SEP clients. We have a small standalone network with ten PCs that doesn't connect to ours. We've installed SEPM on it's server and SEP on the PCs. Obviously we can't update it, so now we've been connecting the public network cable so it can get it's updates weekly. My question is, can we setup the small network SEPM to connect to the main network SEPM to get it's updates instead?

Not really. You could setup for failover/load balancing or replication but this would combine the environments. You could download the JDB file to update the SEPM manually

Ok. Does the JDB file update AV only or everything?

JDB only contains AV defs

How to update definitions for Symantec Endpoint Protection Manager (SEPM) using a .jdb file

Article:TECH102607

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Created: 2007-01-08

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Updated: 2012-06-28

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Article URL http://www.symantec.com/docs/TECH102607

 

Also, see this specific to your current setup:

How to update content on a Symantec Endpoint Protection Manager that doesn't have Internet access

Article:TECH104893

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Created: 2008-01-08

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Updated: 2011-01-24

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Article URL http://www.symantec.com/docs/TECH104893

 

SEPM to SEPM update is possible if you are replicating these two SEPMs. Those will be called replication partners.

Here is the document for that

http://www.symantec.com/business/support/index?page=content&id=TECH104986

Or

Install Liveupdate administrator on a machine which has internet access. Point your sepm to Luadmin

 

Installing and Configuring LiveUpdate Administrator (LUA)

http://www.symantec.com/business/support/index?page=content&id=TECH102701

replication should be able to achieve it, however its not good to replicate the content as some time bandwidth  is the concern.

If you are already considering SEPM replication (not sure there is any sense setting up a separate SEPM for small number of machines), why don't have a look at GUP possibilities - with Group Update Providers you can update AV defintions and all other defs as well:

 

Best Practices with Symantec Endpoint Protection (SEP) Group Update Providers (GUP)

http://www.symantec.com/docs/TECH93813

Group Update Provider: Sizing and Scaling Guidelines

http://www.symantec.com/docs/TECH95353