I have another explanation (sorry, English is not my mother tongue).
I prefer this one, unfortunately it leads to a stronger inconsistency of the scenario:
One should notice that spacetime where Boris meets Jeffrey Price is different from spacetime where J meets Jeffrey. This happens not only a few days after! Actually universe split in two versions in 1969, exaclty when 2012-Boris (old Boris-1) arrived in 1969.
In the first version, in 1969, K blasts Boris-1' arm and sends him to prison in 1969, and 40 years later, Boris-1 escapes from moon, and finally visits Jeffrey.
But in the second version, in 1969, 2012's old Boris-1 kills K before installing the arcnet, so that the young Boris keeps his arms and doesn' go to prison: Boris-2 appears! Consequently, 40 years later, old Boris-2 doesn't need to visit Jeffrey since K is already dead 40 years ago! (Moreover absence of arcnet enables a boglodites' invasion). Actually, the old Boris-1 vanished in 1969 in this second spacetime just after having killed K: in the 2nd spacetime, there is NO Boris without arm who spent years in jail! For a given spacetime, and given age, there is only one Boris (although there may be at the same time two different versions of the same character at DIFFERENT ages, which happens in the movie.)
The good point is that J transited in 2012 from 1st version to 2nd, due to his presence in 1969 (according to Jeffrey). This gives birth to a 3rd timespace version, the one depicted in the second part of the movie.
To sum up, in the 2nd version, Jeffrey should have not met Boris, but only J! Thus no problem if there in only one time device! Nevertheless, there is a big script error. Jeffrey-2 should have never met Boris in 2nd spacetime. Anyway, that inconsistency was necessary for the scenario (By the way, maybe you know that the storyline writing was a huge pain in the ass _ Griffin would have said ^^)
Boris-1 loses his arm in 1969, lives 40 years in prison, jumps in 1969, kills K and vanishes.
Young Boris-1 splits to Boris-2 at the time he keeps his arm (thanks to Boris-1), he doesn't go to prison, and coordinates boglodites' invasion in 2012, as one can watch before J travels to time.
Young Boris-1 also splits to Boris-3 (due to J's intervention upon Boris-1s'one), and is killed by K a few minutes after killing J's father, instead being stopped as B-1 was.
That's not all! Just before vanished, due to J's intervention too, old Boris-1 splits to Boris-4. He dies incinerated by the rocket.
Boris-1 and Boris-4 are not the same! Old B-1 killed young K, not B-4 (dont' forget K is alive at the end).