I did review the current TWU contract and I found it to be remarkably liberal in permissible part-time back-to-back shifts. Pretty much not allowed within a 9-hour stretch (4 & 4, 4 & 5, or 4 & 4 with one hour gap); however, 5 & 5 are permissible even as 10-hour full-time shifts are allowed. Overlapping shifts are permitted so long as they are more than 30 minutes. Given the push for more part-time shifts by the Company, the TWU language makes it remarkably easy to use part-time in place of full-time lines.
Even if the Section 6 goes against the LUS, they may argue they were able to get their healthcare for another 3-6 years, so that's a victory to them.
Sounds much like the IAM fleet restrictions, but they are workable. Although 30 minutes overlap for trades is permissible which makes sense from a company's operational needs for uninterrupted coverage.
LUS allows only two back-to-back full-time doubles due to safety and fatigue issues. 30-minute overlap policy is in writing (I think) as it doesn't require Management permission. Most trades (swaps) required to be completed 24 hours in advance as to provide adequate time for accurate staffing rooster (although it has been commonly allowed with written Management approval, but discouraged). To be sure, day-of trades are disallowed, thus swaps to cover sick-outs between trading partners aren't common.