for i in con_* ; do fslswapdim -z -x y con_$i_swapdim &; done
But you must ensure your con_* list is not too heavy as well as the load generated by one fslswapdim process.
By the way you should also check that their are no restriction about running your fslswapdim process in parallel.
Which shell are you running?
enclose your $i with double quote
Since i don't know about your command fslswapdim, i assume you use it with the correct arguments...so if you are not sure about it, double check it
Give a try with:
for i in con_* ; do fslswapdim "$i" -z -x "$i"_swapdim &; done