According to the Unicode Standard v. 6.1, section 13.4, one of these two new glyphs is used for contextual shaping.  When two consonants occur in Berber with no vowel between, the symbols for those consonants are combined; this is indicated by placing U+2D7F between them.  The Unicode Standard gives examples of what five of these combinations look like, but I'm quite certain that it is not an exhaustive list. If Tifinagh coverage is to be deemed complete, we will need additional PUA glyphs and substitution rules to invoke them.

Additionally, when a double combination of U+2D4D or U+2D4F occurs (these glyphs being made up of vertical lines), the second of the glyphs in the pair is back-slanted. We currently do not have any handling for this eventuality either; we'd need a slanted version of each of these two glyphs and substitution rules to invoke them.

Therefore, at this time I consider FreeSans's Tifinagh coverage to be rudimentary.