Here is another way, using formulae in the rows that picks up the desired
values from columns.
First I assume that the numbers 1,2,3 etc are in col 1 and the letters a,b,c
etc are in col 2
Then starting in col C
in row 1, I enter the formula =row() in each cell. This produces an array
3,4,5 6, etc
in row 2, I enter these formulae (again starting in col C) =3,=3, =C2+1,
=D2+1 etc. This produces and array 3,3,4,4,5,5,6,6,7,7,8,8 etc.
in row 3, I enter the formula =INDEX($A:$A,C2). This produces an array based
on the values in column A, but repeating each number ie. 1,1,2,2,3,3 etc
in row 4 , (starting at col D) I enter the similar formula but for column 2:
=INDEX($B:$B,D2). This produces an array a,a,b,b,c,c,d,d,e,e etc.
in row 5, I select either the number from row 3 or the letter from row 4,
depending on whether the column is even or odd. This is done with the
formula (starting at col C again): =IF(ODD(C1)=C1,C3,C4).
Row 5 now has the desired array 1,a,2,b,3,c,4,d, etc
(commentary - if(odd() = () is a crude way of ascertaining if a number is
even or odd. There may be a better way.)