A two-party system means there are two strong parties and each has wide support of the people. So any party can form the Government after a parliamentary election. It is not just about having an opposition party to check or oppose the Government.
Having enough good candidates is very important, if not how can you contest all the constituencies. At present, how many parties can field good candidates for all the constituencies? Only the PAP. So how to have two-party system.
If we want a two-party system, first the opposition parties should combine together. Then they should actively recruit good talented people to join them. To get people to join them, they must have a good manifesto and tell people what policies and programmes they would implement if elected.
If people think these policies and programmes good for the Country and would benefit the people, I believe good able people would consider joining the Opposition. Even those intending to join the PAP initially may switch to the Opposition.
Otherwise, the oppositions will just remain as oppositions, even if they win two GRCs which I think is possible. They would lose the GRCs at the next election if their MPs fumble. There won't be a two-part system.