Is it possible to ftp from one virtualbox to another running on the same host?
I'd like to drop a file from a Ubuntu VM to a Debian VM, both sit on a Win7 host.
Yes, as long as
No, if one of the VMs has only host-only and NAT adapters. Then you'd have go via the host machine (if both have a host-only adapter) or via an external machine (if both have a NAT adapter). You could set up the host or external machine as a relay (by installing an FTP proxy on it), but that is probably more complication than you care for.
If you have guest additions providing a shared folder on both sides, you can copy via the shared folder. It's slower but requires no setup.
You could create a shared directory on the host that both VMs have access to. I have done this, it works well.
An easy way to move files between two computers when there are compatibility or connectivity issues (obviously some communication will be required) is to open an IM chat between the two and drop the file onto the local IM client. I've done this a couple of times to recover files from clients' old machines that lacked USB, ethernet, or removable-anything in common with the available more modern computers.