multiple vendors AV on single user device

hi 

I have a requirement to have two layers of AV on a single user device.  I already use a different vendor to sophos for the primary solution but was wondering if sophos would be a good pick for the secondary check.

Obviously not all AV products can sit together on a single windows desktop and may cause performance problems

my thinking was to have the second product as a command line only solution, this being called from a script after the first vendors sweep has executed.

I know sophos has a command line tool and I hope this could be used, before I can test this I have the following questions...

1) How would a command line only sophos solution stay up to date signature wise, alot of command line only AV products are for a standalone  environment and do not have a backend infrastructure.  I would prefer a pull solution from the desktop to check for dat updates ETC, I don't want our sys man solution pushing this.

2) Is the sophos signature digitally signed?

thanks all

fox