Hi Takeshi,
Sorry for the delay
In theroy, you can use ;
LIGATION_SITE = GGACC,GGTCC
I think it should work. But honnestly, I never use it that way.
If you do that I think that checking in details the results of the mapping should be a good idea. Especially, at the end of the bowtie2 step1, when it generates the list of unmapped reads and then tries to trim them based on the ligation motif.
If you find something weird, please let me know so that I could investigate this in details.
Nicolas