There is no need for any speculation here, read the policy guidelines, pg.18 clearly states,
https://www.gov.uk/government/uploads/s ... _07_14.pdf
102. Your overall total earnings can include
those from several sources of work, including
a combination of salaried employment and
self-employed activities.
103.
Earnings do not need to have been
with a single employer.
104. Earnings can be considered from
full-time, part-time, temporary and short-term
work.
As long as you satisfy the earnings requirements and score enough points to qualify, it could be any 6 months or 4 months or even just 1 month earnings (within the 15 months prior to visa extension application date) is enough to claim points for earnings.
In summary, if you submit for 12 months, and they are not happy with the 3 months statements, then they will award points for the earnings based for the rest 9 months, and it doesn't matter if the 9 months earnings are split between 2 employers.