For low strength apps I'd bevel both ends (30*)...with wire I'd leave a small gap...with tig I'd just but them up(laying in an angle iron "jig") and butt weld them.
For higher strenth apps I either build a tight fitting box that slides inside...or use square tube if it fits tight enough.
I usually use plug welds on the inner "Box". Plug weld one end FIRST before you slide the two together. Ummm I remember once.. I slid all three together at once and the inner box support slid also. The one end slid right to where the weld joint was on one end....not good.
Leave a 1/8" gap between the two ends and weld it up hot.
This makes for a very strong joint.
(BTW.. I've done this a lot...scrounging used tubing out of the dump etc)
I have tools I don't even know I own...