Несколько одновременно открытых запросов в одном соединении к PostgreSQL

Я думаю, ты не сможешь. Одно соединение может одновременно обрабатывать один запрос.
Это описано в главе «Асинхронная обработка команд» документации libpq:
«После успешного вызова PQsendQuery вызовите PQgetResult один или несколько
раз для получения результатов. PQsendQuery не может быть вызван снова (в том же соединении),
пока PQgetResult не вернет нулевой указатель, указывающий, что команда выполнена».

Here are some notes on MARS and why I'm not sure it belongs in Npgsql. tl;dr unlike SQL Server, PostgreSQL simply doesn't have a mechanism that would allow an efficient MARS implementation.

One aspect of MARS is to make it easier for users to write ADO.NET code: users can execute multiple commands without caring or tracking if the connection is already in use. This is purely a convenience thing - it's always possible to rewrite code so that it either (a) uses separate connections, (b) buffers the entire resultset before execute another command, or (c) uses PostgreSQL cursors and explicitly fetching results for the different commands.

Beyond convenience, another aspect of MARS is performance. To the best of my understanding, when using MARS on SQL Server, the multiple resultsets are streamed back from the server without any additional roundtrips being made by the client, as is required when using PostgreSQL cursors (e.g. FETCH NEXT). Considering the perf importance of roundtrips, this difference is really essential - an application doing MARS on SQL Server isn't doing any additional roundtrips, whereas a cursor-based MARS against PostgreSQL would be adding a lot of roundtrips. However, if users do want this behavior on Npgsql, they can use cursors themselves and accept the extra roundtrips.

The same is true if MARS is implemented by buffering the entire resultset - users can do it themselves if they wish, but that has a potentially large cost in terms of memory usage.

To summarize, given that PostgreSQL itself doesn't have an efficient MARS implementation (AFAIK), it doesn't seem right for Npgsql to implicitly do MARS under the hood, with considerable perf costs that would be implicit - unsuspecting users would be falling into a perf pit of failure.

MultipleActiveResultSets = true

в строку коннекта добавить MultipleActiveResultSets = true

preload reader=true

есть проект на mssql, там такое встречается, мне его нужно переделать на PostgreSQL.