In your example there is no task that is both the first available and tagged with Today, so it produces nothing. You were hoping it would create a list of today tasks and perform a second level of processing to determine which of those is first available – but I think it only operates on the original project data set.
Other approaches:
Use status: Available instead, and find other ways to cull the list, for instance by adding a status: Flagged. Go and flag certain high priority tasks while adding Today tags.
Or don’t cull the list and keep it sorted in Project Order, so even though multiple Today tasks may be showing, you can focus on the item at the top of the list.
Create a First Available perspective and go through it in the morning adding Today tag to select items. Then you literally have the First Available and Today tagged items. It’s actually the reverse of what you wanted (Today tasks selected from First Available instead of the other way around) but it could be a way to make forward progress on a bunch of projects.
I feel like inevitably have to cycle between perspectives, one showing a broader view from which you can make selections, and another more of a hotlist or agenda of a select few items to focus on. I haven’t found a magic perspective that’s very focused and can auto-update in exactly the right order.
