Hello William,
Im no expert, but I dont believe that there is a way to do as you describe, without using busy lamp field (BLF) shared lines or speed dials. However, if you would like to continue having the same functionality, you can make each phone have 2 lines with the same extension, but in 2 different partitions. Example:
ext 1234 is in Line 1_Partition
ext 1234 in Line 2_Partition
Make a new Calling Search Space called Line 1_CSS. Place Line 2_Partition in this CSS. Then, on DN configuration page, on call forward busy, forward to ext. 1234, and set the call forward calling search space to Line 1_CSS, and the call will roll over to line 2 if line 1 is busy. I hope that made some sense.
Hope that helped, if so please rate.