This is a guide to get this, on your own if you wish, by playing local co-op.
This will also lead into the "Rivalry" achievement as both can be achieved comfortably within 30 minutes. Rivalry is attained first as there are a few occasions when there is only one conversation option (forcing an agree) - this is the most efficient way to earn both achievements I believe but if you only want "Of One Mind," the earliest this can be achieved is still conversation 4.
A couple of things to note is that:
As listed, you must do this in one session. I repeat this as if you make a minor error in forgetting to select an option (quite easy to do at least once if you get the controllers get confused) you cannot reload as it will reset the counter.
Secondly is that you do not need to be controlling the 2nd character at all times; you can have the second account select "leave game" between conversations (although then this second account will not get the achievements).
Finally, if a conversation has started you CANNOT sign in with the other account. You must have player 2 in game before conversations.
Now for the list of conversation points, and how many times you can disagree / agree. Typically, never select the "ending" option, such as "Let's go." These are usually quite clear. (NB. To have the player select their conversation option, move their left control stick to an option and press "A". An icon with "P2" and a tick will appear).
Conversation 1: After defeating the first few waves of enemies within the game, Guiscard will open a door and beckon you over. Turn on your 2nd controller, sign in with a local account and select a player of your choice. Approach the door and a conversation will start. Have player 2 select an option, player 1 select the opposite. Only one conversation option may be picked before moving on. (1 disagree, 0 agree)
Conversation 2: Before leaving the bridge after setting the 3rd barrel keg, have player 2 sign in again (to play safe I always sign in before lighting the last keg). Run off the bridge and
another conversation will start. Ask all questions before proceeding to "Let's go." When there is only one option left after completing one topic of conversation, you can have player 2 simply agree. (9 disagree, 3 agree).
Conversation 3:
After the second save point, before facing your first (mini?) boss. While speaking to the mercenary, again select all inquisitive options while player 2 selects the opposite. (3 disagree, 0 agree).
Conversation 4:
After defeating the first mini-boss (I had player 2 signed in the whole time - but you should be able to have them sign out and sign back in after the fight), head inside to talk to Odo. Again there are different conversation paths you may explore (typically questions). (6 disagree, 2 agree).
Rivalry will be achieved here.
To continue you on for "Of One Mind":
Conversation 5:
Before hitting the gate/quest marker for the first town, Rukkenvahl, sign in with player 2 again and talk to the man at the gate (5 agree).
Conversation 6:
Continue into Rukkenvahl, and find the optional quest giver. I continued through the conversation to receive the quest (not ending the conversation early). (3 agree).
Conversation 7:
Continue through Rukkenvahl until Katarina shows up. Have player 1 pick the same options as player 2 (5 agree, "Of One Mind" achieved).
This is one of my first guides and I hope you found it helpful. Let me know if you need anymore help.