How I think of it is like this: those with Fi and Te determine their values and goals internally, based on their own desires. Thus they operate in a more 1st person manner, thus 1.
Those with Ti and Fe tend to be more problem solving, taking the values and information which are already out there and then processing this, either as an external conflict solving (Fe) or internal analysing (Ti). Thus they have a 2nd person approach, and are denoted by 2.
I think Ni and Se tend to have more of an Adjusting way of perceiving, in that they have general models which they calibrate to the particular situation.
I think Ne and Si have more of a Building mode of perception, coming up with specific ideas, and storing this in a model made from basically static parts.
These distinctions might not be totally accurate, but it is a good way to remember it I think.