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The following letters all have something in common which may not be obvious at a first glance:

A B D H P

No other letters share this attribute.

Hint

There are no misspellings or typos in the title of this question. Maybe a clue though.

More hints may follow if the question is not answered.

Some great answers so far all of which I have upvoted but none of which are exactly what I am looking for.

Hint 2

The number 0 and the character ( also have the same property. I only said no other letters share it ;-)

Hint 3

As correctly identified by @RedBaron the ASCII table is key here. There is a good reason why "sum" is mentioned in the title and there are two reasons why "two" is mentioned.

The property seems to be related to:

Binary equivalents of the symbols/alphabets etc.

Explanation:

Binary equivalents for the following can be written as:

A -- 01000001

B -- 01000010

D -- 01000100

H -- 01001000

P -- 01010000

0 -- 00110000

( -- 00101000

So the property is,

The sum of digits in the binary equivalents is two

Or

The binary equivalents of all these have two 1s and six 0s.
Looking at the binary equivalents of the alphabets, one can see that no other alphabets share this property

The title (Thanks to @trolley813):

Two might refer to the sum of the digits, which is indeed two!
Title might mean that the sum [of digits in] letters is not different from two [but is equal to two]

Old (and wrong) answer

The property is:

The index if each alphabet is equal to the sum of indices of the preceding alphabets in the sequence +1.

And,

There is no other alphabet with the index 1+2+4+8+16+1 = 32

The property is that

each of their alphanumeric values (A=1, B=2, C=3...) is a power of 2.

Is it

All the letters, symbols in this group have ASCII codes of form $2^m + 2^n$ where m and n are integers

Thus we have

From ascii code table,
$A = 65 = 64 + 1 = 2^6 + 2^0$
$B = 66 = 64 + 2 = 2^6 + 2^1$
$D = 68 = 64 + 4 = 2^6 + 2^2$
$H = 72 = 64 + 8 = 2^6 + 2^3$
$P = 80 = 64 + 16 = 2^6 + 2^4$

Other letters don't share this property because

64 + 32 = 96 which does not correspond to any letter. The letter a begins at 97

For the newer hints

$0 = 48 = 32 + 16 = 2^5 + 2^4$
$( = 40 = 32 + 8 = 2^5 + 2^3$

I think it's:

Each letter's alphanumeric value is double its predecessor, which also means, sum two times the alphanumeric value of the previous letter

This means that:

Starting from A=1 we get the sequence 1,2,4,8,16,... which corresponds to the sequence A,B,D,H,P