In and of itself it would probably just prolong the inevitable. At this stage the Sassanids were in terminal decline and had been in a near constant state of civil war for some time now (
just look at the number of Shahs that are listed from 590 onwards). Their military was exhausted, Arab accounts of them marshalling hundreds of thousands of soldiers are likely self-aggrandisement, and the Shah was constantly putting down rebellions. The Sassanids were ripe for the taking which is why they collapsed so easily in the face of the Muslim conquests.
An alliance with the Byzantines might be able to stave off their total collapse long enough to stabilise. It's not likely, the Byzantine–Sasanian War of 602–628 was less than a decade ago, but desperation and mutual enemies have been known to turn sworn enemies into close allies.
It's hard to tell where things go from there. I think that the Sassanids would remain the sick man of the region for a while and will depend on Roman support whilst the Muslims will likely have the upper hand for a while. Once things stabilise they could either keep up the alliance with the Byzantines or attempt to play the Byzantines and Caliphate against each other.