Walking my 2 dogs is an ordeal, which makes me not want to do it. I have a 70 pound lab and a 30 pound border collie. Can I use a 2-way coupler to walk dogs so different in size? Also would it be possible to use a retractable leash with a coupler, or do I need to use a standard leash? Should I get a 12 inch or 24 inch coupler- does it make a difference? Thank you.