I am planning to buy some US ETF, as well as some SG ETF. Should i open a local brokerage only or both a local and overseas brokerage to save on currency conversion?

These days, opening an account with a broker can be done completely online with just a few clicks. That shouldn't deter you from using different brokers - I personally use 4 different brokers, each with their own role for investment. You can see my answer to another question here where I explain what you should look out in a broker.

Many of the local brokers here charge ridiculous commission fees, forex spreads and other types of handling fees. Also note that these fees go both ways - you will incur them again when you eventually sell your investments. Do a bit of searching for brokers that meet your needs and you will save lots of money in the long run.