This article deploys a qualitative comparative analysis between Bangladesh and Sri Lanka on constitutions and parliamentarism. Both these South Asian countries began independence with a Westminster model of government. Bangladesh exhibits some successes with parliamentary design which could be replicated in Sri Lanka. Therefore, the study explores Bangladesh's key lessons in parliamentarism: The role and appointment of a ceremonial president; political accountability; floor-crossing; parliamentary committees; and Bangladesh's Technocrat Ministers. Based on John Stuart Mill's most similar cases principle well known as \"method of difference\", this study select case studies that control for variables as far as possible, by being as similar as possible on everything other than the factor those are researching, then that is the causal factor in any diverging outcomes. The findings suggest that Bangladesh's innovations in political accountability, parliamentary committees, and Technocrat Ministers which could be replicated in Sri Lanka. Also, Sri Lanka should design a presidential balancing mechanism. On the other hand, floor-crossing provisions in Bangladesh should probably not be a feature of the Sri Lankan system. Therefore, this comparative analysis helps Sri Lanka to avoid pitfalls and recreate successes that other countries have already experienced.