Given the following :
-A Maga may invent many spells during a single seasonal activity as long as all the spells have the same TeFo ;
-ArM5 core rulebook say you simply have to stack all the spell levels.
Then I made the following assumptions :
-She is allowed to invent two spells with distinct requisites, for example a CrAq 10 and a Cr(Re)Aq 5 as long as she has a Laboratory total score of 30 (two times 10+5)
-She is not allowed to invent two spells who only have common requisites, for example CrTe(Aq) 10 and Cr(Re)Aq 5 ;
-Given the requisites (or even Lab specializations, minor magical focus...) may increase or decrease each spell's Lab total, I assume each spell's Lab total now must be calculated separately.
Or, if the spell levels permit, you could divide the season in 2 half seasons (dividing your lab total accordingly) or one third/two third of a season. I could swear I read this rule somewhere but I can't find it
No. You can arguably do this when you read a book, but lab activities cannot be split (as callen points out).
Instead, any deviatetion from your main lab activity that takes up more than 10 days (total) penalzes your lab total.
As for requisites, if you apply the standard rules literally, you do get a solution that makes sense.
The "basic" Arts must be the same for all spells involved (in the example, Creo and Aquam).
The spell levels are added together (in the example 10+5=15).
In the presence of requisite Techniques and/or Forms, the magus must use the lowest Technique involved, and the lowest Form involved (in the example, the magus uses the minimum of his Creo Aquam and Rego Aquam lab totals: if both equal or exceed 30, he can invent both spells in a single season).
This deals with everything from lab specializations to Magic Foci.
FWIW, I think it would break nothing to ignore the Arts involved, given the above.... but them's not the rules. (Indeed, such a House Rule would encourage magi to learn more low level spells rather than fewer high level spells, which opens some character concepts and explains why some magi leave their apprenticeships with diverse spells.)
No splitting seasons either for reading books or lab work. You can get interrupted though.