Ok, we had differing views on this during a game last night.
A unit destroys its H2H opponent. This leaves it in a position to count as a support to the next combat.
Now I assume, that the rules saying each combat is fought in order and in the order chosen, means that results in one can alter another. The counter opinion was all combats would be simultaneous so they would be counted as in combat and unable to support.
The order you fight the combats in is up to theActive player. If a unit wins the combat and decides to stay where it is, it can support it's neighbour. It only needs to be in an eligible position at the point of combat resolution. The combats are not simultaneous. If they were, then a unit of cavalry that could Sweeping Advance, would not be able to charge into another unit and fight another round immediately, which it can, even if the charged unit is engaged but hasn't fought yet.
im not sure it would, ok as a noob i probly get this way wrong but i thought supports must be worked out before combat starts as each unit can only support or take part in a combat once per turn. so two units with 1 behind them are in combat the unit behind can only support one unit and if that unit wins it cant support the other as they are kinda busy. the principal of all combats been simultanious
No, any units within 6 inches of a fighting unit are eligible for support. They only come into effect when determining combat resolution. There is no need to declare anything, you just add the appropriate units to the scores when deciding the winner. So if I score three hits on your unit and I have two supports that would count as five toward combat resolution, although the supports are not "hits". Also, if any units are in a position to support and are not used, for instance you have two units within 6 inches of your left flank, one behind the other, and your unit breaks, then both of those units must take a break test. This is because either could count as supporting even though only one did. Re-read the rules on supports and break tests on page 71, Paragraph 4.
building on that last answer, if two units side by side are fighting enemy units to their front and one breaks in its combat, does the other have to break test as a 'theoretical supporting unit' within 6 inchs, even though at that point, as an engaged unit, it was not eligible to support?
No it doesn't. The reason is because the remaining unit is fighting and not supporting. Supporting units don't fight. When a fighting unit breaks all eligible supporting units must test, whether they are used as support or not. A unit that is fighting only tests if it loses combat or becomes shaken and the combat is drawn.