The best way would be to speak to someone near computer A
This is the issue with dual boot machines, you need to be there to make the relevant choices.
One alternative is to use virtual machines (I use vmware at work and Virtualbox at home)
This saves having to reboot a physical machine to use a different OS.
If this was a HP Proliant Server you could use Advanced ILO to power cycle the machine and view a remote console not sure if there are similar options for the home market.