The wiki states that "holding shift allows you to ignore the block boundaries when placing a chiseled block", but this doesn't work in game. Instead, the block stays red and will not place. The wiki is not remotely in-depth and does not cover another way to do this.

When you press 'R' or whatever your hotkey is for the selection circle thingy, press the arrow in the middle and there should be an option called "Merge"! Assuming you're probably on version 1.20 :]