Merging cells "only if" second cell is populated

col A and col B has data

resultant col formula

 =IF(NOT(ISBLANK(B1)),CONCATENATE(A1,":",B1),A1)

Thanks Wizard,
Your solution worked for the columns that both have info (filled in from both sides with colon in center), but I also need to have the third column filled even if it is just from one column but without the colon inserted and nothing after it.  (Examples:  if two columns both contain info, need both info and semicolon;  if only column has info - need that info to come into new column "without" a semicolon after it.)

The solution i posted does give you the values in the both columns with : at the middle when both the columns are populated.
If there is not data in 2nd column, only the first column data is in the result (without :)

Can you send me the excel sheet for sample please. cant get your point.

I appreciate your assistance.  I cannot send sheet because info does not belong to me.  I ran your formula and the only cells populated in the new column are cells where info came in from both of the other columns and are separated by the semicolon.  If there was only info in one of the initial columns (not both) then the cell in the new column is empty.  It worked perfect.  But I need ALL cells in the new column populated even if there was only info from one column moved to it.

What i am trying to say is "if two cells are merged and there was info in both then I need a semicolon inserted.  If two cells are merged and info is only coming in from one column then I need that info but will not require the semicolon.

column B will always have info; column A "may" have info.  Sorry, should have mentioned that in the beginning.

Example of sheet.  Need column C populated in all cases.
Wizard.xlsx

BINGO!!!!