A range can't span multiple worksheets.
This may work, depending on what you ultimately need to do:
Set newRng = Worksheets(6).Range(range1.Address, range1.Address)
But, since these ranges have the same address in your example, I think what you want is not a Range object combining them, but some other data structure, like an array, collection, or dictionary.
NOTE Your ranges are not valid to begin with, Range("A2:A") is not valid, so you'll need to fix that. See here for reliable ways to find the "last" cell in a range. I've modified it to bring in the entire column A (except A1) but you will probably want to fine-tune that.
newRange will have to be a different data type for this to work without raising a Mismatch error, for example a Collection:
Sub Combine()
Dim coll as New Collection
Dim range1 As Range
Dim range2 As Range
Dim ID As Range
coll.Add Worksheets(3).Range("A2:A" & Rows.Count)
coll.Add Worksheets(4).Range("A2:A" & Rows.Count)
Set newRng = coll
End Sub
Or as an array of range:
Sub combine()
Dim newRange(1) As Range
Set r1 = Worksheets(3).Range("A2:A" & Rows.Count)
Set r2 = Worksheets(4).Range("A2:A" & Rows.Count)
Set newRange(0) = r1
Set newRange(1) = r2
End Sub
Using the array example above, you can then assign the values to another location, modify as needed:
Worksheets(4).Range("B1").Value = newRange(0).Value
Worksheets(4).Range("B2").Value = newRange(1).Value