Assuming that we are still speaking on a (no-serial-diode) matrix keyboard, you are able to detect multiple keypresses in a column and/or row, if thee is no common pressed key between multi-key columns and rows.

Check it this way:
- draw a similar picture as you did before, mark the desired combination grey. Draw a red line onto each column with more than 1 pressed button, draw a green line for "multi-button" rows. Where red and green lines cross, there must be no grey at those points.

Jan Waclawek