-snip-
Are you sure you are using the correct s and z in the second equation? You say you have k, and you should only have one r value if k is being reused, so it should simply be a case of taking s
2 and z
2 and using them to calculate the second private key.
z1=0xf8fa613cf79cb88f337138799056babe30ae6c8d6b55df46de00f01bc5ba6f55
r1=0x896324077e81aa06ee4b61d2f7d769a7726fd1b0d468c7599038e9c279954b2a
s1=0x675c4bb39f1667bc66e06a9b47fe9b59f6936b6677fcc8d3b884d43b80370216
x=0x60a2d645cdd5a3ec97b2be8fec30ee936ed390116afa324ec2426d805f08927c
k = (z1 + r1*x)/s1
k scalar:0x252258e92cbac2ef388abfd2301c62a9cc9d2bc708fc80ad363af54934673fc
-----------------------------------------------------------
z2=0x91cdde4574e71a505eb956aadd6ba7f700adf9a50fed676b0eddcbb589e438c0
r2=0x896324077e81aa06ee4b61d2f7d769a7726fd1b0d468c7599038e9c279954b2a
s2=0xd99dd8fc7d32cdf025cba3bd80ed51a6dbf860da29eb506c40016f22a19efc42
k=0x252258e92cbac2ef388abfd2301c62a9cc9d2bc708fc80ad363af54934673fcc
x2 = (k*s2 - z2)/r2
x2 scalar:0x60a2d645cdd5a3ec97b2be8fec30ee936ed390116afa324ec2426d805f08927c