I got it to working. The problem I was having is that I had the wrong address for start for the other cores. Once I fixed the address for the start everything worked.
srec_cat BuoyOperMulticore_Z4_0.bin -Binary -offset 0x00000000 -fill 0xff 0x00000000 0x00005000 BuoyOperMulticore_Z4_1.bin -Binary -offset 0x00005000 -fill 0xff 0x00005000 0x0000A000 BuoyOperMulticore_Z2_2.bin -Binary -offset 0x0000A000 -o combined.bin -Binary