[William Cloud Hicklin]

However, I would think that that occurred very late, after Saruman went entirely bad. As a member of the White Council Saruman naturally would have known about the Gladden Fields just as Gandalf did.

Numbers: The Chronology, which with luck will be published before long, casts a lot of light on Enemy movements. It's fairly clear that although on the one hand G and U were nominally 'cooperating', it's also the case that Sauron's original plan was to intercept the Company at Sarn Gebir, keeping Ugluk well out of it. After the ambush failed, G crossed the river but with only a small part of his Orcs, because crossing the River at the Rapids was not easy or quick, and because he couldn't leave the east bank of the River unguarded. In the meantime, Ugluk in the Emyn Muil had not only his Isengarders, but a substantial party of Mountain-orcs he had brought down from Moria. In other words G was substantially outnumbered, and thus was forced to feign cooperation and simply keep an eye on Ugluk, orders which were reinforced by the second Nazgul when he broke away for 'consultation.' (Note that G left Ugluk's party by himself; the forty Uruks he returned with were apparently new reinforcements. Whatever Orc-commandos he had crossed with apparently were killed at Amon Hen, or by Ugluk in the Emyn Muil. The forty new guys one would think were 'picked Orcs,' since their forced march/marathon to the eaves of Fangorn was the most astounding feat of any of the parties crossing Rohan: 100 miles in 35 hours.)