In terms of a Ximenean puzzle, it is a 'no-no'. In 'Ximenes on the Art of the Crossword', he writes, 'two consecuive unches...this is not allowed'. Looking at the broader context, in the Chambers Crossword Dictionary, Jonathan Crowther (Azed) says, 'In [blocked and barred puzzles] the inclusion of consecutive unchecked letters in answers is considered bad practice and generally frowned on.'
One could perhaps argue that an eight-letter entry with only two consecutive unches (the other six letters being checked) was 'more' acceptable since the increased difficulty of solving would be balanced out by the fact that as many as three separate unches would have been permitted. But given that in a six or seven letter answer a maximum of two unches are allowed, that argument fails. If you are planning to submit the puzzle for publication, I would definitely advise steering clear of consecutive unches...