Limit data entry into one of two cells and restrict to one value in one of the cells

[John EPC]

Here is what works for the first part: to allow entry into cell A1 only if Cell A2 is blank or opposite. Leave user of spreadsheet with choice of cells to enter but not allow entry into both.

select cell A1, go to data > validation

select custom, in formula type;

=IF(A2="",TRUE,FALSE)

and in A2 do the same but change the formula to;
=IF(A1="",TRUE,FALSE)

How do I go about adding limit value to cell A1 to 143? Hopefully, it can be done by adding to the formula used in cell A1 in Data Validation. If it can not be done that way, how can it be done using Visual Basic for Applications?

Thanks, John