I've tried 7.0 version as secondary using swap and restoring a TWRP backup after, and no way with any of two methods. The only way to run nougat with mutirom (as I know) is installing it as primary. But the secondaries won't start up.
Maybe multirom is not ready to nougat... yet :).
Try changing this
block_image_update("/dev/block/platform/msm_sdcc.1/by-name/system", package_extract_file("system.transfer.list"), "system.new.dat", "system.patch.dat") ||
abort("E1001: Failed to update system image.");