what's the best way to join two parts

Hi,

for a file,

$ cat a_3.txt
aaa
bbb

123
321

what if I want to join two parts together, the output wish to be

aaa 123
bbb 321

I only can think of one way of doing it,
put
aaa
bbb
in one file b_1.txt
put
123
321
in another file b_2.txt
and then
paste b_1.txt b_2.txt

are there some way not need to output as two files,

Thanks ahead for any advice,