Generally, while having multiple accounts is acceptable, any interaction between them is forbidden. This includes especially anything that allows you to do things which would not be possible with just one single account, but not only related to direct reputation inflation.
Even answering questions of your alt or commenting on their post can be seen as abuse of the system, as it e.g. allows you to accept your own answers and get points for it, or to express opinions on your post without making visible you're the author.
Quoting Shog9 (SE Community Manager) on How should sockpuppets be handled on Stack Exchange?:
When should sockpuppets be considered a problem?
There are a handful of legitimate reasons to maintain multiple accounts. A good rule of thumb for identifying abusive socks is: if the second account allows you to do something on the site that your normal account would be prevented from doing, it is abuse. Examples of this include (but are not limited to):
- Voting on your own posts or comments
- Answering your own questions with the other account(s)
- Casting multiple votes on others' posts or comments
- Supporting your own arguments ("+1: shog is right, don't know why the rest of you don't realize this")
- Using bounties to circumvent the rep cap
- Circumventing suspensions, quality bans, or the rate limits on posting questions / answers / comments / etc.
Also note that your alt accounts are in danger of getting merged with your main or even deleted if they cross certain lines and trigger automatic systems or moderator attention. So it's best not to step too close to that border anyway.