Threads in PSQL

Dear postgreSQL experts,
could you explain how should I run correctly multiple threads that
manipulate data from the same database?
In my program, I have main thread that SELECT  large object's id and
starts a thread that should export this lo to some file. Each thread
creates it's own PGconn connection, and starts lo_export(conn, loid,
filename) procedure. The problem is: after starting 2-3 threads SELECT
query of the main program  fails because "another command is already in
progress". Since main program does SELECT queries only, then it means
that this query conflicts with a command issued by one of the threads.
Again, I use individual PGconn in each thread, but the line for
PQconnectStart(const char* line) is the same for all threads as well as
for the main thread. Should I create connections using individual port
number? Or there is some other way to make my threads really independent?
Thank you, Sergey.