Originally Posted by dsgtc0408

To the OP: go ahead and ask the Duty Officer anyway. As shuigao suggests, there might not be an issue. If Singapore ICA can accept the two passports then I don't see what loss there is to you other than spending a few minutes to speak with the official. eIACS doesn't just bypass the lines of foreigners waiting their turn to be stamped into the country, it also saves the trouble of filling out the entry card as well. eIACS is also at the other border crossings so its value isn't limited to the airport. Unfortunately I don't see any email address where you can contact Singapore ICA on this question so you can only ask in person.

Separately please keep one thing in mind when visiting foreign countries given you can choose between the US or ROC passport. If you run into any legal difficulties during a visit to any foreign country, you can only get consular assistance from the country whose passport you used. Any other country that you have citizenship with has no legal standing to assist. You happen to be visiting Singapore this time. If you use the ROC passport it's a ROC representative that would contact you, but Singapore's relations is with the PRC. If you use your US passport or potentially eIACS, then it's US consular officials that would get in contact.

I'm bringing this up because all of us are focused on convenience in all aspects of our lives (and why you asked about eIACS) but there's potentially real consequences to the choice of citizenship we declare when we enter a foreign country.