Multibox Positioning/engaging?

Is it possible to make certain chars stand x yalms away from target?
also how would I let melee/tank chars engage and approach target?

Yes can use keyboard/mouse or a controller to move character. Can use distanceplus addon to track yalms from target.

Yes

select the target
then use your aattack button /key

you don't do anything differently when running multi character whatever it is singleboxed or multiboxed, the game works the exact same way.

Keep in mind that it will be difficult to get your characters x yalms away from the target and also for your melee characters to approach their target if they're under the effects of gravity. If your characters are affected by bind, it will be impossible to accomplish these things.

One last thing nobody has mentioned yet: if you are stunned or terrorized, you will not be able to engage with your melee characters.

Good luck and let us know if there are any further questions, the community here is really helpful.