I think it reasonable that a given druid circle worships the same deity. They might also show respect to other nature gods, but the circle itself venerates Sylvanus (on this occasion) above al the others. A circle, after all, is like a temple. Other druid circles on the Sword Coast will, no doubt, primarily worship other deities.
it is reasonable, but the opposite is just as reasonable. There are many instances in the forgotten realms of temples dedicated to more than one god, most famously the temple of Corellon's Grove in Evermeet, which, despite it's name is also dedicated to Sehanine, Aerdrie, Hanali, Labelas, and Titania and Oberon. In regards to druid circles, their tendency to mix worshippers of various deities is long established in the lore.
