A function  can only return a value or values. It can't change any cell
directly, just the one(s) that is is returning value(s) to. You could make your
function an array function, so that it can occupy multiple cells. For example:

Public Function boomLength(radius As Double, height As Double)
Dim vData(1 To 1, 1 To 2) As Variant
vData(1, 1) = (radius ^ 2 + height ^ 2) ^ 0.5
vData(1, 2) = "insert text  here"
boomLength = vData
End Function

Then, if entered as a simple function, it returns only the boomLength value.
But if array-entered (i.e. Ctrl-Shift-Enter) as a row and two columns, you'll
get both the boomLength value in the first column and the text in the second
column.

When you say "Both lines of code  work separately ..." I doubt that this is quite
the case. Certainly, the "Cells (1, 2)" line  doesn't work in my Excel (2000)
even if I comment out the "boomLength =" line.

There is nothing wrong with the "Cells (1, 2)" line itself. If I put it into a
separate subroutine, it works  fine. It's where you've got it that is the
problem. The function  you have defined is returning a value for insertion in a
cell. That is all it is allowed to do. It can't plant a value in a different
cell. Excel terminates execution of the function at the point it tries to do
something illegal, and execution continues with whatever was running when the
function was invoked.

In summary, functions called from cell  formulae can't change other cells.

To make it simple, I would advise you to do the following:

In the procedure that's calling  the function, I would create a variable that
will hold the result of your function
and then I would concatenate in the following way
Worksheets(1).Cells(1, 2) = "The text  you want to insert  " & yourVar