You already have implemented methods for look for documents (cmd+o) so why not use second input to search for document and have unlimited secondary inboxes?
Having secondary inboxes still require additional actions, you need to choose one from a list. I donāt know how you implemented recent change in file finder, but maybe itās possible to copy this mechanism to show (under the second input) documents which were used recently as inboxes. I would myself go even further and use the same mechanism as in āmove itemā to attach item under chosen document or item, that would be even more flexible.
Unlimited inboxes could be handled quite simply I think. When a user presses Ctrl+Shift+I, and the thought box pops up, below it, or next to it, would be a list of other inboxes, sorted by most recent usage, and the default inbox that pops up is the most recently used one. Holding Shift and clicking could allow you to send to multiple inboxes, and clicking once would send to the new inbox, and that would then become the most recent one.
Rather than a fixed number permit an arbitrary number with a pop up listing them if there is more than 1. The most recently added could go at the top of the list, or the end of the list. Need a way of removing them alsoā¦
I donāt really see the disadvantage in secondary inboxes as pinned drop downs below the capture box (exactly like the move dialogue) - there is all that empty space, why not use it?
Or indeed like this person suggested - typing and pressing enter sends to primary Inbox, but there is an additional optional dialogue box to choose a list like the move dialogue to send the item anywhere you like (and then perhaps with the 5 most recent choices below just like the move dialogue)
One concern might be distraction. When you want to capture some thought that can be easily forgotten as it comes up, seeing any extra text in the app foreground might be a distraction.