I am not understanding what you mean.
right column +2
Left column +1
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).
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.