Too easy! I just set mine up like this. Depends if they are on the same account or not. If all toons are on on the same account it is really easy!
1) Create a HBRelog Profile (select Toon 1 and your garrison profile)
2) Create a Wait task under that HBRelog profile - most garrison profiles will finish in < 20 minutes even with a level 3 mine and garden
3) Create a Logon task under that HBRelog profile - select Toon 2 and your garrison profile
4) Repeat steps 2 and 3 for each additional character
I have toons on different accounts:
Same as above, but instead of creating a logon task, you have to chose load HBrelog Profile and have a 2nd profile for the other account.
That's it!