It is possible to lose both accounts, yes. If the accounts are linked by CC/Email/Account Info, or if you are mailing the fish/money between them Blizzard may ban both accounts for 'exploitation of the economy'. Depending on how severe they feel the fish botting is, and what kind of mood the AGM (account gm) is in, you may get a 3 day vacation, or sent up the river permanently.
I don't know how long bot investigations are taking these days, but I would recommend not doing it for a while. In the past it took Blizzard about 3 weeks to investigate, and you never knew when a GM was going to be watching you. So get out and fish normally, play normally. Be normal.