One spouse can be the primary TV holder and can write a sponsorship letter for their spouse for the second application. Ideally you'll show financial stability, more than likely if it's a joint account a million baht would be satisfactory. The Canadian consulates are very responsive so I'd put your question to them and as far as the GIC I doubt it.
There is no certain time, but, immigration is cracking down on people using back to back visa exempt stamps, it also depends on which border you're using and either flying in or crossing a land border.