[One (M8) Android Development] Captain_Throwback:
The only way I've been able to get any of the CM12-based ROMs to flash as a secondary is with this updater-script, which I got from an earlier version of BlissCM:

assert(getprop("ro.product.device") == "m8" || getprop("ro.build.product") == "m8" || getprop("ro.product.device") == "m8wl" || getprop("ro.build.product") == "m8wl" || getprop("ro.product.device") == "m8wlv" || getprop("ro.build.product") == "m8wlv" || getprop("ro.product.device") == "m8vzw" || getprop("ro.build.product") == "m8vzw" || getprop("ro.product.device") == "m8whl" || getprop("ro.build.product") == "m8whl" || getprop("ro.product.device") == "m8spr" || getprop("ro.build.product") == "m8spr" || abort("This package is for device: m8,m8wl,m8wlv,m8vzw,m8whl,m8spr; this device is " + getprop("ro.product.device") + "."););
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "/system", "");
package_extract_file("system/bin/backuptool.sh", "/tmp/backuptool.sh");
package_extract_file("system/bin/backuptool.functions", "/tmp/backuptool.functions");
set_metadata("/tmp/backuptool.sh", "uid", 0, "gid", 0, "mode", 0755);
set_metadata("/tmp/backuptool.functions", "uid", 0, "gid", 0, "mode", 0644);
run_program("/tmp/backuptool.sh", "backup");
unmount("/system");
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/userdata", "/data", "");
package_extract_file("system/bin/otasigcheck.sh", "/tmp/otasigcheck.sh");
package_extract_file("META-INF/org/cyanogenmod/releasekey", "/tmp/releasekey");
set_metadata("/tmp/otasigcheck.sh", "uid", 0, "gid", 0, "mode", 0755);
run_program("/tmp/otasigcheck.sh");
sha1_check(read_file("/tmp/releasekey"),"7241e92725436afc79389d4fc2333a2aa8c20230") && abort("Can't install this package on top of incompatible data. Please try another package or run a factory reset");
unmount("/data");
show_progress(0.750000, 0);
block_image_update("/dev/block/platform/msm_sdcc.1/by-name/system", package_extract_file("system.transfer.list"), "system.new.dat", "system.patch.dat");
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "/system", "");
delete("/system/bin/otasigcheck.sh");
unmount("/system");
show_progress(0.020000, 10);
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "/system", "");
package_extract_file("system/bin/backuptool.sh", "/tmp/backuptool.sh");
package_extract_file("system/bin/backuptool.functions", "/tmp/backuptool.functions");
set_metadata("/tmp/backuptool.sh", "uid", 0, "gid", 0, "mode", 0755);
set_metadata("/tmp/backuptool.functions", "uid", 0, "gid", 0, "mode", 0644);
run_program("/tmp/backuptool.sh", "restore");
delete("/system/bin/backuptool.sh");
delete("/system/bin/backuptool.functions");
unmount("/system");
show_progress(0.050000, 5);
package_extract_file("boot.img", "/dev/block/platform/msm_sdcc.1/by-name/boot");
show_progress(0.200000, 10);
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "/system", "");
assert(run_program("/system/bin/makelinks.sh") == 0);
ifelse(is_substring("0P6B20000", getprop("ro.boot.mid")), run_program("/sbin/sh", "-c", "busybox sed -i 's/ro.com.google.clientidbase=android-google/ro.com.google.clientidbase=android-verizon/g' /system/build.prop"));
unmount("/system");

However, when doing so, mobile data doesn't function at all, for an unknown reason. So the best bet is to flash it as primary, backup, and then restore that backup as a secondary. I'm sure the issue is in the scripting somewhere, but with the new zip layout I'm not sure exactly what the fix is.