Yes you can do it. If your using betfair it all comes down to which side you have less selections to handle. If your in a 10 horse race and 6 are lays then bits easier to back 4 selections. If your only going to lay 2 horses then its easier to lay 2 selections.

But it can also come down to %s as well as doing laying 8 horses in a 10 horse race will not make as much as backing the 2 horses. Same the other way around, if you only lay 2 horses then laying will generally give better returns then backing the 8 horses.

If your talking on the TAB then you need to be removing at least 15% (or be getting a rebate to offset the % takeout). In that case just backing will work providing you are removing enough selections to make a profit.