This use of the Grapheme Joiner is not recommended:
From Unicode Standard, Chapter 16.2 Layout Controls - Grapheme Joiner
Rendering For rendering, the combined grapheme collector is invisible. However, some older implementations may process the sequence of grapheme clusters connected by combining grapheme joiners as a whole for applying enclosing combiners. For more information about grapheme clusters, see Unicode Technical Report No. 29 "Unicode Segmentation Text". For more information on enabling combining labels, see Section 3.11, Normalization Forms.
From chapter 3.6 Combination
It should be noted, however, that older implementations may have supported the use of a spanning combining label for the whole Index of a consonant conjunction or sequence of connected grapheme clusters together, combining graph collectors. This approach has a number of technical problems and leads to interoperability defects, so it is highly recommended that the implementation does not execute it.
And from http://unicode.org/faq/char_combmark.html#19
Q: Is it possible to apply a diacritic or combining mark to a sequence of more than one (non-multiplying) character?
A: No, except for the "double diacritics" deliberately intended to be applied to a two-letter sequence, for example. U + 035D COMBINED DOUBLE SHAWS. Neither ZWJ (U + 200D ZERO WITDH JOINER) nor CGJ (U + 034F COMBINING GRAPHEME JOINER) "glue" the characters together so that they are influenced by the volume of any subsequent matching character. To get a character sequence, such as "Esc", into something like U + 20E3 COMBINING ENCLOSING KEYCAP, you must resort to higher-level protocols. [KP]