Hibernate is your only option and eventually you will need a fresh reboot and lose the stuff.
Hibernation is a literal save of the memory to disk followed by a full system shutdown. It also has an extra step of saving device state.
Unlike sleep, it doesn't require power. It is truly a shutdown.
When the computer is booted next, windows checks that the ...
Add (create) a NAT Network within Virtual Box
Connect each of the two VMs to that NAT network
Make sure that each VM has a unique MAC address
if you have statically assigned IP addresses, make sure each VM has a different IP address within the 10.0.2.0/24 ip range
if you want to ping VM B from VM A via its host name, add the host name of VM B and the ...