This makes it very unlikely that you'll be able to hit high AC enemies or make difficult saving throws. The chance of passing a saving throw that would normally require you to roll an 18, 19, or 20 on a d20 is 15%. The same chance on 2d10 is 6%, worse than half. Same thing with hitting opponents and making skill checks. You'd have to seriously up the CR of enemies with very high ACs or difficult saving throws, or edit them, or not use them.
Another example: If you would normally need a 20 to hit an ac 50 monster, that is 5% chance of success. If you roll 2d10, you only have a 1% chance to hit the same monster.