new Exchange 2010 setup

I am setting up a new Exchange 2010 environment. Here is what I have:

One Active Directory domain with 300 users
Two separate external email domains.
50 users are adb.com domain, and the other 250 users are xyz.com domain
The adc.com domain must use a smart host to send mail through.

I wanted to setup one mailbox server for the first company and one mailbox server for the second. Then use one HUB and CAS server for both companies. I have discovered that I cannot setup separate send connectors in Exchange 2010 without third party software.

So….do I buy the third party software, or change my design?

One MB/HUB/CAS server for each company
or
One MB server for each company and one HUB / CAS server for each company

Thanks,