Optimising fermentation space

What are the dangers of keeping wort (1,2,3 days?) from multiple brews to eventually put together in a bigger fermentation vessel and optimise the space? If everything is super hygienic, is it inevitable that something will spoil it? Any microbiologists out there?

Not a microbiologist, but well versed in food safety and this is pretty similar. If the wort were boiled and chilled and racked to a vessel in a sanitary manner, sealed tight and placed in a cold environment (under 42 F - the colder the better), you'd likely have very low risk of contamination from your process thus very low risk of bacterial growth.

The residual risk here is that, even with only a few colonies of a bacteria carried over during transfer, the pH in wort is not low enough to completely inhibit the rapid and progressive growth of those microorganisms, but the cold temperatures will limit the types of bacteria that could survive, hence "the colder the better" above.

Never done experiments along this line and I don't have hard data so personally; I wouldn't go very long before fermentation. I can't imagine, under strict control as above, you'd get nasties in 3 or 4 days - I suspect it'd take a bit longer than that.

Another risk presents itself when you bring the wort back up to pitching temps. You're letting it enter what is referred to as "The Danger Zone" (between 42 F and 150 F, roughly). If there were any critters introduced during the initial transfer, this is when they'd wake up and have breakfast. Best to get the yeast in there as quickly as possible (at the correct pitching temp, of course) to provide plenty of healthy competition and shut those beasties down.

Or you could start the first fermentation and add extra worts to that later. Probably require much less yeast.

Brewers sanitize, not sterilize. Sterilization is 1000x fewer bacteria than sanitizing.

Raw wort is a great growth medium. We just want out yeast to dominate. And it does so, very well. Like all microflora, it wants to create an environment that is hostile to anything that isn't itself. It does this by scavenging all the air, creating CO2 and not least, producing enzymes and ethanol.

A well pitched wort, that kicks off in a timely manner (12 hrs or less) has such an overwhelming yeast population that the relatively small population of other microbes largely get beaten to death by fermentation. What's left, usually doesn't have sufficient strength to do any damage to the finished beer under normal circumstances.

In a non competitive environment, bacterial will consume nutrients, protiens, starches, and sugars. Even at cold temperatures, they're inhibited, but not stopped. In such close proximity to nourishment, they will have plenty of opportunity to prosper.

In Lambic brewing, the time between entering the coolship and when fermentation begins is prime time for a lot of microflora that won't survive fermentation, but do leave a mark on the wort.

Acetobacter prefers ethanol, but will consume other wort elements, like dextrin. This critter is able to double its population every 20 minutes under the right circumstances. It will wait out fermentation before beginning its tour through your beer, if the population is sufficient. Long timeframes between chilling and pitching are good for acetobacter.

Even if you're going to blend the worts together, it's best to get SOME yeast into the wort. Even if it's just a pack of dry yeast.