So instead of one system.ext4.tar you may end up with system.ext4.tar, system.ext4.tar.a and system.ext4.tar.b
In this case you can simply use Linux command CAT (concatenate) to merge them into one piece.
Here I have broken pieces of cache, data and system.
I will only use boot.img and system.ext4.tar for the ROM cooking, so no need to merge cache and data here.
If you try to set the working folder with the Kitchen, it will say ext4 is invalid since it's 0 byte.
In Cygwin Linux environment, type this command. * is a wild card that represents all files start with system.ext4.tar.
cat system.ext4.tar* > system.ext4.tar
After concatenate is over, you will have system.ext4.tar file with updated file size. You can see the sum of tar.a and tar.b is same as output file.
Remove tar.a and tar.b, start cooking the ROM.