I am not understanding what you mean.

You say???????

right column +2

Left column +1

Then:

Right column is 19 (3 lines and 4 dots).

Your question - can only add one.

Answer - no - you can add the 2 to make 21 - leaving only 1 on right column (one dot + one carryover to left column).

You say:

Left column has 2 already + 1 (carry over from right column) + 1 from floor = 4 dots.

Is that what you mean?

-------

Tell me what you have on left and right column and also what you have on the floor.

I'll see if I can explain it better with what you start with.