I think you will need more than one IP since each unit will require port 5060 and 20000 and 20001 to communicate.
You can either forward the port to one unit or the other.
So as far as I can see you will have to order two IP’s set your dsl device to bridge mode then connect up a small 4 port hub, put a NAT router (only needed for security reasons) for each ATA (one for your lan and one ATA and the other for the other ATA)
Direct port 5060, 20000 and 20001 on each linksys to the connected ATA device.
You should use two separate Deltathree accounts if you intend to receive calls.