There is now a Share Army button available, even in Missions L.1. Unfortunately, it doesn't work.
Oh nicely spotted :D
This is like a preview of what's to come. In the future we'll be able to "share units" so that alliance commanders can control them. I don't believe this is related to your question of timing however.
Basically, you can consider that one battle round takes place every 1-2 seconds. That should give you an idea of how good your timing should be if you want to synchronize.
Also I remember this wasn't clear for me, but you can consider battle rounds to be synchronized with the rest of the world, ie. a battle takes some real-life time (see above), and if additional units arrive while a battle is taking place, they will join the fight for the next battle round.
The total duration of the battle depends on the attack strategy (plunder/onslaught), the size of the army, and charges. For example if you send rams against a city without garrison but towers, your army will stay for 40 rounds (because rams only have 40 charges), so you can expect like 1min of battle. If an army arrives "in-between", it will not generate another report.
Note that it becomes weird if the attackers have different strategies. For example, an army sent on onslaught will leave as soon as there is no unit/building to kill. For this defender scenario, and if at the same time as the onslaught army, another army on plunder arrive, well the onslaught army will just leave after one round while the plundering army will remain to plunder.