2 columns to 1

If your data starts in row 1

Formula:

=IF(ROW()>COUNT(A:A)+COUNT(B:B),"",IF(MOD(ROW(),2)=1,INDIRECT("A"&ROW()-INT(ROW()/2)),INDIRECT("B"&ROW()-INT(ROW()/2))))

If your data starts in row 2

Formula:

=IF(MOD(ROW(),2)=0,INDIRECT("A"&ROW()+1-INT((ROW()+1)/2)),INDIRECT("B"&ROW()+1-INT((ROW()+1)/2)))

If your data starts in row 3

=IF(MOD(ROW(),2)=1,INDIRECT("A"&ROW()+2-INT((ROW()+2)/2)),INDIRECT("B"&ROW()+2-INT((ROW()+2)/2)))

Modified and eliminates 0s at end of data. I have used full column references that should be shortened to be suitable for your data:
Start in Row 1:

Formula:

=IF(ROW()>COUNT(A:B),"",IF(MOD(ROW(),2)=1,INDIRECT("A"&ROW()-INT(ROW()/2)),INDIRECT("B"&ROW()-INT(ROW()/2))))

Start in Row 2:

Formula:

=IF(ROW()-1>COUNT(A:B),"",IF(MOD(ROW(),2)=0,INDIRECT("A"&ROW()-INT((ROW()-1)/2)),INDIRECT("B"&ROW()-INT((ROW()-1)/2))))

Start in Row 3:

Formula:

=IF(ROW()-2>COUNT(A:B),"",IF(MOD(ROW(),2)=1,INDIRECT("A"&ROW()-INT((ROW()-2)/2)),INDIRECT("B"&ROW()-INT((ROW()-2)/2))))

Start in Row 4:

Formula:

=IF(ROW()-3>COUNT(A:B),"",IF(MOD(ROW(),2)=0,INDIRECT("A"&ROW()-INT((ROW()-3)/2)),INDIRECT("B"&ROW()-INT((ROW()-3)/2))))