Hi,
BubbleUPnP cannot play synced audio to multiple renderers that it detects. This is technically impossible (the syncing).
For playing to 2 linked AudioCast devices, the group would have to appear as a single UPnP AV renderer in BubbleUPnP.
Such functionality would have to be implemented by the AudioCast hardware and software, but since you do not see a single renderer
for the linked devices, it is probably not implemented.