root@qjw-VirtualBox:~# seq 0 9 | sed 'N;s/\n/ /'
0 1
2 3
4 5
6 7
8 9

root@qjw-VirtualBox:~# seq 0 17 | sed -e '1~3N;s/\n/ /' -e 'N;s/\n/ /'
0 1 2
3 4 5
6 7 8
9 10 11
12 13 14
15 16 17

root@qjw-VirtualBox:~# seq 10 19 | sed '1~2 s/1/x/'
x0
11
x2
13
x4
15
x6
17
x8
19

root@qjw-VirtualBox:~# seq 10 19 | sed -e '1~3 s/1/x/' -e '2~3 s/1/x/'
x0
x1
12
x3
x4
15
x6
x7
18
x9

#!/bin/bash
next_flag=0
last_line=""
cat tmp |
while read line
do
	if [ "${next_flag}" -eq 0 ]
	then
		if echo "${line}" | grep "aaa:" > /dev/null
		then
			last_line="${line}"
			next_flag=1
		fi
	else
		if echo "${line}" | grep "bbb:" > /dev/null
		then
			echo "${last_line}"
			echo "${line}"
			next_flag=0
		else
			last_line="${line}"
		fi
	fi
done

root@qjw-VirtualBox:/tmp/test# ./tmp.sh 
aaa:2
bbb:3
aaa:6
bbb:7
aaa:10
bbb:11
root@qjw-VirtualBox:/tmp/test# cat tmp
aaa:0
aaa:1
aaa:2
bbb:3
bbb:4
bbb:5
aaa:6
bbb:7
bbb:8
aaa:9
aaa:10
bbb:11
aaa:12