Merging cells in multiple columns on multiple sheets
Some notes -
Sub or Function
Functions should be used when something is returned and subs should be used when something happens. In this case they are actually both Subs.

Naming
So I think PIS is the actual name of the sheet, not a variable. But, it's all uppercase and not very descriptive. In this case I would think it's a constant because
Standard VBA naming conventions have camelCase for local variables, UPPER_CASE for constants and PascalCase for other variables and names.

Variables
You did a good job declaring all of your variables, but it's always a good idea to turn on Option Explicit. You can have it automatically by going to Tools -&gt; Options in the VBE and checking the Require Variable Declaration option. This way if you have any variables not defined, the compiler will let you know.
Names - names are cheap and characters are free. You can use this to your advantage by giving all of your variables descriptive names. For instance - RngB tells me nothing about what that range is supposed to be doing.
Integers - integers are obsolete. According to msdn VBA silently converts all integers to long.

Select Case
Why do you have a select case given you're basically using it like an IF? It looks pretty messy, to me. You could fix that up with a constant. Personally, I'd pull it out as its own function -
Private Function IsTargetSheet(ByVal sheetName As String) As Boolean
    Const SHEET_NAMES As String = &quot;Med Curr, Med Ren, Med RevRen, Med Prop, Med Renewal Alts A, Med Renewal Alts B, Med Renewal Alts C, Med Prop Other Markets 1A, Med Prop Other Markets 1B, Med Prop Other Markets 1C, Med Prop Other Markets 2A, Med Prop Other Markets 2B, Med Prop Other Markets 2C, Med Prop Other Markets A, Med Prop Other Markets 3B, Med Prop Other Markets 3C&quot;
    If InStr(1, SHEET_NAMES, sheetName) &gt; 0 Then
        IsTargetSheet = True
    Else: IsTargetSheet = False
End Function

And now you just need this in the main sub -
For i = 1 To sheetCount
    sheetName = ActiveWorkbook.Sheets(i).Name
    If IsTargetSheet(sheetName) Then MergeCellsx sheetName
Next i


MergeCellsx
In case you didn't know, a Sub can take an argument the same way as a function -
Sub MergeCells(ByVal sheetName as String)

I see you are targeting columns 2 and 3, 5 and 6, 8 and 9. When you have something like that, it might be better to put those number in a variable or constant so that if they change, you can just change the initial value of the variable. -
Const FIRST_NAME_COLUMN As Long = 2
Const SECOND_NAME_COLUMN As Long = 3

Now every time you type in the columns, you can use the constant which will tell you what the column is supposed to contain.
Also, in this instance, you would probably be better with a long if instead of 15 conditional loops. Maybe even the select case would belong here. When you see repeated code like that, it's an indication you can refactor it.