Thanks for the quick reply!
I was told the stipulation for a $399 discounted price for a second lifetime service required the other Lifetime subscriptions to be full price, not a discounted lifetime subscription.
My first lifetime subscription was $99 for an older Tivo HD.
It makes sense that I wouldn't be eligible for two discounted lifetime subscriptions though, but the Tivo website used to offer me a second discounted subscription in the "Upgrade Offers" section.
Oh well, it was a little too good to be true