can i put together 2in conduit x10 ft... joined together x2 be used as a flag pole totaling 20 ft strong enough?

Yes. Did that exactly. Added cap on end to keep water out. Sunk it into ground 3’. Added rocks and concrete mix around it. Hasnt moved in 6 years. Also painted it and added pulley and rope before putting it up.

We put a 4' piece (2"ID roughly 2 3/8" OD) concreted in place (12" diameter Burke Tube flush with the ground) with about 4" (2" pipe) sticking out of the ground. Screwed two 10' joints together with a truck and ball at the top. With help you screw the two joints (previously joined with a coupler) onto the "stub" sticking out of the ground (Coupler) and tighten it securely. Ours has been in use with a 4' x 6' flag for over 30 years. Withstands high winds but if the wind gets above say 30 mph we take the flag down. Purchase truck and ball from a flag supply company such as Carrot Top. The truck has the pulley that you thread the rope through before attaching the two joints to the concreted in "stub." The clip that the halyard (rope) tightens onto can be bought at the same supplier or Home Depot sells these as well. In calm winds we also put a smaller 3x5 flag below the larger 4x6. In 25 mph plus winds we take them down. Screwing the 2 joints onto the "stub" allows for taking the pole down for maintenance such as rope changes or repairs to the truck or ball.

CAN you join two 10 foot lengths of 2 inch rigid steel conduit with a coupling? Yes, you are able to do that. Will it be strong enough for a flagpole? Maybe, but that isn't the use it has been UL Listed for.