How to group lines in file by two?

With awk:

awk 'NR!=1{print x"-"$0}{x=$0}' file

• NR!=1 applies on all line, except the first one
• print x"-"$0 print the values with dash between
• x=$0 set x (for the next iteration)

With POSIX sed:

sed '1{
  h
  d
}
H
x
s/\n/-/
' <file

or one-liner version:

sed -e '1{h;d' -e\} -e 'H;x;s/\n/-/' <file

paste -d- - ./infile <infile

^That would work really well, except that your input is off-by-one. So...

{ echo; cat <infile; } | paste -d- - ./infile | sed '1d;$d'

...would work, but maybe is too complicated...