My bike goes up one gear at a time, but down two: is this normal?
On or off road, but especially off road, it is desirable to be able to shift down more than one gear at a time to deal with abrupt changes in gradient and avoid being stuck in too a high a gear and stalling out.

Also (as you mentioned) if you are required to slow down or stop suddenly, it's convenient to be able to drop down several gears while simultaneously braking, then be in an appropriately low gear to accelerate back up to speed.

Some modern shifters allow downshifts of 4 or 5 gears. Yours may provide 2 with a full stroke of the lever, so the solution may be to simply 'half stroke' the lever to get a single downshift.

If you edit your question to add the make and model of shifters and derailleurs you have we may be able to provide a more specific answer (or tell if there is something wrong).