Wow. I have 5 kids, one with special needs, and one on the way very soon. I have breastfed all of them exclusively and never kept a logbook for nursing/sleeping or even thought about it. I have tracked my son who has special needs poop schedule- it's needed because of his paralysis issues- but I have not even kept up with writing it down, I just keep a mental file on how he's doing. Also in all my years of attending La Leche League meetings I have never heard it recommended or discussed...
Why would you need to do this? Do you have known milk supply issues caused by breast reduction surgery? If not, if I were you I'd save myself the trouble and spend the time I might have spent logging such things taking pictures of my babies, spending time with them, or getting more time for myself. Especially with twins, you'll need some "me" time!
I know this doesn't answer your question, but I can't help but think it's a reasonably "good suggestion," given that humankind has survived for millenia without mothers having to keep logbooks...
Mothering Multiples by Karen Gromada is supposed to be a good resource for nursing twins, but since I haven't had twins I've never read it personally.. Congratulations on your pregnancy and good luck on your parenting journey!