Is it possible to have two collector instances on one box?

I don't think this is a good idea but if the collector does limited work... Most (all?) of the WMI checks will take credentials via the wmi.user and wmi.pass, so perhaps you can flip it around: Setup the LogicMonitor windows services to use the SQL account but then add the wmi.* properties with the monitoring service account details.

Still think it's a bad idea, would suggest 2 collectors also, but perhaps if the situation is very limited it might make sense. This is something that is also useful when you are dealing with multiple AD domains.