Gloria Exercitvs With One Standard

It doesn't seem to have a meaning. The ones with two standards are usually less ornate - perhaps theres a meaning to it lost to time? You often find the single standard issues with labelled banners.

I thought it might have to do something with when Rome was divided into two, east and west

The two standard variety is earlier and larger; the one-standard is later and smaller. The switch happened, according to RIC, about 335. It is certainly sometime after 333 when Constans became Caesar, because he can be found with two standards, but rarely.

I do not have a scholarly reference to hand right now that answers the question, but I think it marked a change in value. Harl (Coinage in the Roman Economy, 300 BC to AD 700) says "Sometime between 330 and 335 when the nummus fell to an AE4 module coin, the state and public began reckoning nummi at 1000 to the solidus." He says this while documenting the continual increase of the number of nummi per solidus until the FEL TEMP REPARATIO reform of 348. He does not answer our question, but it seems he thinks the weight of the coin mattered in reckoning its value, and then I deduce the smaller 335-337 GLORIA EXERCITVS would be worth less. Would the mint be honest enough to mark the lighter coins differently--with one standard?
Here is an example of Constans with 2 standards, followed by one with one standard. The original photos are to the same scale and show the former is substantially larger (18 mm vs 16 mm), but this site enlarges my smaller picture to the same size as the larger so you can't see the difference easily.

When the two standard GE was being issued, there was a ratio of 132 coins per pound. In A.D. 335, the number of nummus to a pound was raised to 192. The coins needed to be smaller, so there was only room for one standard.