Will try to answer, staryeyed:
"If Ds2 6 wks doesn't feed so much one day does that affect the next days supply?"
Not so you would notice, no....supply is driven by the removal of milk from the breasts, so if on one day for whatever reason, your baby feeds less often, he is not 'driving' the supply as much, but I can't think you would actually be aware of a drop in supply...in any case, if the baby needs to feed more the next day, he will simply feed more, either at each session or by feeding more often. He will get the milk he needs that way, and in addition, drive the supply more.
"If he feeds madly how long before the extra milk comes in?"
If he takes a lot of milk, he leaves less milk behind, and the replacement milk is made more or less on the spot. Established, effective bf (which is a description of your bf, I would guess) is very responsive - some milk is made as and when the baby feeds, and some of it is made in response to the emptier breast, between feeds. The rate at which milk is produced varies - long gaps between feeds = slower production; short gaps between feeds = quicker production. It is exactly like a fast food restaurant - their production rate gets faster during a rush, and slows during a lull.
"Are breasts ever empty"
No - unless bf is going badly for some reason.
" and if not why does Ds2 stop feeding on a side and take the other one?"
Because flow slows down and the baby wants a faster flow; because he wants the less creamy milk at the start of the next side; because he wants a break and a burp....because he wants to be turned the other way....whatever
"If DS2 normally has both boobs does it matter if he switches between them when feeding?"
No. He will just adjust his intake to meet his needs anyway.
"Should they be encouraged to have both sides even if they seem happy with one- would the supply adjust to feeding from one breast per feed?"
In the beginning of bf, or if there is any suggestion of the mum's supply needing a boost, or the baby's intake needing a boost, then offer both sides each time. Two sides increases volume of production, and gives more opportunity to the baby to increase intake. But if bf is going well and is well-established, and the baby is happy and thriving on one side only, no problem. Supply adjusts, downwards.
"Does supply work from feed to feed or over a period of time"
Well, both, really.
"- if not feeding well at night time can that reduce supply"
Yes it can - now this may not be an issue with an older who starts feeding less at night. He may have less need for calories because he's having solids, and he may also have a mother with a robust milk supply anyway. But in the early weeks and months, long gaps between feeds could be crucial- not as a one off, but over days or even weeks.
"Would boobs then adjust to the needed day time and night time supply?"
Not really - breastmilk production is very generous at night in the early days and weeks because prolactin has a diurnal/nocturnal pattern, but this is not very marked at all as time goes on. In fact, prolactin has less and less of a role in breastfeeding and milk production as bf becomes established as a 'supply-demand' process. I think you are saying 'can breasts tell the time?' and so produce a lot in the day and less at night, and the answer is really 'not in the way you mean'. There is always milk in the breasts. The clock has very little to do with this.
Hope this helps - ask again, if I haven't answered clearly enough