I am using a modded version of the 'Legions' mod to enable two units per hex.
[PLOT_UNIT_LIMIT, Set Value="2"] This generally works fine except when combat situations arise where either I am sharing a hex with an enemy or an enemy is sharing a hex with another faction (The second can screw completely if you try and attack getting stuck in a 'Do you want to go to war' loop.
I have read through this thread and it would seem that nobody has yet found a solution to the combat problems. There also seems to be no definitive answer as to whether this IS fixable or is likely hardcoded.
A possible solution would be to prevent units of different factions sharing hexes (without preventing ' Pass throughs')?? Although this would seem to leave the question of which unit attacks and defends in a two stack situation?
This is starting to be a game-breaker for me. Personally I find 'normal' Civ5 (1 unit per hex) way too restrictive and think it causes more problems and frustrations than it solves. With 2 units per hex many problems (and virtually all frustrations) are solved (defending catapults and generals, moving where YOU want without units suddenly swapping places or being blocked unnecessarily), BUT and it's a big but, once combat starts to get in any way involved (with more than one unit per hex) the whole thing falls apart and more than likely I will sooner or later encounter either mysteriously invisible units, a major infinite query loop or a 'waiting for opponents' loop that means you have to reload.
So I'm in a can't live with - can't live without it situation. (i.e. A game breaker). A solution would be SOOOO great. An answer to some of the questions posed in this response would be a start I guess.
(It seems to me (as an old ex programmer myself) that the game was written to allow multiple units per hex but that the 'programmers' were unable to solve the combat problems within budget and time constraints and therefore went for the 'easy' option of allowing 1 unit per hex.)
I'm also starting to think that if this was solvable then some of the expert modders out there would have cracked it by now.