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.