Is sharing one paid account with one neighbor permitted?

Well, it's not going to work, as the words or characters you need to review are 100% personal to you. If you shared the account, your neighbour would see the words you need to review, add words that you don't want to review, do your reviews for you, probably the effect would be neither person would know much of anything.

It has to be one account, one person for practical reasons...

You would have to make completely separate lists and never use the "study all option," which is the most efficient and in my opinion the most useful feature of Skritter. The statistics would also be useless because there would be no way to differentiate between both of you.

For saving $5/month that definitely isn't worth it. If you're hurting for an extra $5, I'd suggest spending more time getting your finances in order before using that time to study Chinese.

I'm not taking offense, but for some of us, our finances are in order. We just don't have a lot of money and studying Chinese happens to have a higher priority in the budget. So $5.00/month can be important. However, I don't think sharing an account would be worth the trouble even if it were possible. And it isn't as Byzanti and Josh have pointed out.

I agree on this. But, I'd be interested to know if there is any link between Chinese and Japanese on Skritter. Can two people share if they are using different languages?

Maybe time to refer a few friends and get them to sign up so you get a few weeks free
skritter.com/referral

Not that anyone I've referred has signed up yet - slackers

In theory, yes, you could have two people study two languages on Skritter as the are separate from each other. Though you wouldn't be able to study at the same time.

There are cookies which might get screwed up with two people using one account.

It would work for one person to study Japanese and one Chinese on the same account, if you didn't mind sharing certain settings which aren't language-specific. But progress data would work. You would also not be able to study at the same time; you'd have to coordinate study times, since it'll get all messed up if you're practicing Chinese and then your neighbor switches you to Japanese mid-session.