only pops up the account that is already selected, where I'm sure in the past it brought up both accounts and allowed me to select the other account to send the email
Make sure that your "second" account actually has the outgoing (sending) mail server configured.
For this please long press on the account on the app's main screen -> account setup -> manual -> next.
You'll see detailed outgoing (sending) server settings.
There is a checkbox to not have an outgoing server (rare, but valid, and some people need that) - please make sure you've not enabled that by accident.