Before starting this, you might want to take a look at Chapter 3, Date and Time Functions, in the iWork Formulas and functions User Guide. You can download the guide via the Help menu in Numbers. You'll also want to see Formatting Date and Time values in the Numbers '09 User Guide, available in the same location.
In general, I would avoid splitting or merging cells in Numbers.
The easiest method of combining these columns might be to add two new columns, then enter a formula to concatenate the contents of the MDY columns in one and a formula to concatenate the HM values in the second. An example of one of the formulas in listed below.
Copy the result, then go Edit > Paste Values to replace the formula with the concatenated values.
Repeat for the H and M columns.
Then delete the old columns.
If your MDYHM data is in columns B, C, D, E, F the formula for the Date column will be =B&"/"&C&"/"&D
Be aware that the results in each case will be a text value, and can't be used in calculations.
If you want actual Date and time values, check the F&F guide, and post a reply if you need further assistance.
Regards,
Barry