Here's a quick one in 84 (updated from 86 because I had a pointless !! before eof) (unless I miscounted) -- relying on $^F defaulting to 2:

#--------1---------2---------3---------4---------5---------6---------7
+---------8--------
#234567890123456789012345678901234567890123456789012345678901234567890
+123456789012345678
perl -anF, -le'$h{$F[0]}|=$^F;$^F-=eof}{"@_"=~/[12]$/&&print"$&:$`"whi
+le(@_=each%h)' t1.csv t2.csv
1:8 
1:6 
2:1 
2:3 
2:5
[download]

As always with hashes, I can't promise anything about the order of the output, but that seems to be okay. :)

Update:
79, by using 2-@ARGV instead of $^F
78, by using the implicit print from -p
77, by splitting manually instead of with -a

#--------1---------2---------3---------4---------5---------6---------7
+-------
#234567890123456789012345678901234567890123456789012345678901234567890
+1234567
perl -pe'@h{/([^,]*)/}|=2-@ARGV}while(@_=each%h){$_="@_"=~/[12]$/&&"$&
+:$`$/"'
[download]