Welcome to our new Microsoft Q&A Platform.
“ Laptop must be capable of connecting to two domains, one for CompanyA and one for CompanyB”
One laptop can only have one domain, when you join CompanyA, you must exit CompanyB.
Simple and best way is to partition the hard drive and install the OS on each partition. Then set each OS to connect to a single domain. When Booting up, select the OS relavent to the domain you want to connect to.
You can still share information if you need to but this also keeps everything separate. It's just like walking around with two different laptops.
Besides, you could setup a virtual machine on the laptop that is a member of the other domain... or have the laptop a member of a workgroup and two virtual computers so you can have one for each domain.