The statement from "ark" is not completely true. This doesn't imply that I am an expert here too.

I had the same level of information like - Only one OPT at the same level of study.

But I know 2 people who did their bachleors twice in two different fields and got the OPT both time.

They graduated as Chemical Eng and later got the Bachleors in Med Tech.

So............there is a work for you to do my frd.