Code:
$
$ cat f7
SH1,QC,12334
RD1,MO,898909,35476
SH1,BC,34556
RD1,FG,2341212,909090
SH1,TR,787878
RD1,GH,12345,676767
SH1,YO,565656
RD1,GO,7878604,23978
$
$ ## using Perl
$ perl -lne 'BEGIN{undef $/} s/\n(RD1)/,$1/g; print' f7
SH1,QC,12334,RD1,MO,898909,35476
SH1,BC,34556,RD1,FG,2341212,909090
SH1,TR,787878,RD1,GH,12345,676767
SH1,YO,565656,RD1,GO,7878604,23978
$
$ ## using awk
$ awk '/^SH1/{printf "%s,", $0} /^RD1/' f7
SH1,QC,12334,RD1,MO,898909,35476
SH1,BC,34556,RD1,FG,2341212,909090
SH1,TR,787878,RD1,GH,12345,676767
SH1,YO,565656,RD1,GO,7878604,23978
$
$