Center punch location heavy and square: drill 3mm pilot hole thru. first wall of tubing. Use a dab of cutting oil, or any oil on drill bit. If it starts to squeak then add more oil. If you are producing hot swarf then you are too fast, keep constant pressure on, do not allow drill bit to skid. A variable speed drill makes this job a whole lot easier. Re-align drill square for drilling thru. back wall of tube and repeat. Open up the two holes with your 10mm drill bit, using oil again and keep the speed down and constant light pressure on but not too heavy, the drill bit will jam if you have too much pressure, when it breaks thru. Take it slowly.
From an ex-toolmaker. Hope the above helps.
Running around like a head with it's chicken cut off.