Differential Backups with Media Offsite

Hi,
I have a well-established and proven backup strategy using other products, and need to understand if I can reproduce it using Acronis TrueImage.
Presently I do a full backup of all my systems every couple of months. This is a lengthy process, and takes 1-2 days to run, so I cannot afford to do it more frequently. Once the backup is complete I take the media offsite.
On a weekly basis I do a differential backup. Since I use ntbackup to do the differential backups this does not need to reference a full backup archive, and the differential backups are quick and fit on a much smaller portable disk. I then take this offsite.
I have two complete sets of media. At any time the set offsite has the latest backups, and the set in my own office is effectively unimportant, although it does provide a "back stop" older backup if required.
To restore any files I need a maximum of two disks (the last full backup and the last differential backup), although in many cases the last differential backup alone would be sufficient.
I have tried to reproduce this using Acronis TrueImage, but the problem is that the differential backups seem to need to refer to a full backup image before they can create a differential one. If this is not available (and in my model it would be offsite) the backup process attempts to create an entire full backup image each time, which will be completely impractical with the voumes of data on my systems.
Is there any way to create a differential backup relative to a backup archive which is not currently online, or am I forced to continue to use older backup technologies in order to run an effective onsite/offite model?
Thanks
Andrew