I strongly recommend using more variables. Your code might look like this:

// Height is different in multicell, this stands for lineheight and should be the same as other cells :)
// I would advise you to go for, SMALL_CELL and BIG_CELL if you want to differentiate
$WIDTH_CELL = 60;
$HEIGHT_CELL = 5;

// Most of the time, you want to border all so let's put this in a variable as well 
// Not in your case though, but more on that below this :)
$BORDER = 1;

$pdf->Cell($WIDTH_CELL, $HEIGHT_CELL, 'MAILING ADDRESS', $border, 0);
$pdf->Multicell($WIDTH_CELL, $HEIGHT_CELL, $puchaser1[0]["address"], $border);
$pdf->Cell($WIDTH_CELL, $HEIGHT_CELL, $puchaser2[0]["address"], $border);

$pdf->Cell($WIDTH_CELL, $HEIGHT_CELL, 'PHONE NUMBER', 1, 0);
$pdf->Cell($WIDTH_CELL, $HEIGHT_CELL, $puchaser1[0]["contact_no"], $border, 0);
$pdf->Cell($WIDTH_CELL, $HEIGHT_CELL, $puchaser2[0]["contact_no"], $border, 1);

However, this cannot solve your problem. You're going to have a bad time because you need:

1. Track your starting Y
2. Save Y value after adding multiple cells
3. Use these two values ​​to draw a rectangle for the entire row

All other borders should be specifically "L", "R", or "LR". You have to draw the top border, bottom (sometimes L or R) border with a rectangle.

There is a lot of code, so if you have such code, I will help you further :)