Sadly not - the Gatsby cat harness is designed for use with a single leash, with an attentive human being at one end! This is important for two reasons:
Escape-proofing: The Gatsby cat harness has several escape-proofing features that are specially designed for cats walking on a leash with their human. Because the angles and forces are so different when tethered (vs. walking on a leash), there is an increased risk of escape.
Safety: Unsupervised tethering can be potentially very dangerous for your cat. Whilst on a tether and unsupervised they can become entangled in their tether with very serious consequences.