It’s possible to get an MS in BioE/BME after a BS in Biology, but you would have to take a significant amount of additional math and engineering coursework either during or after your BS if you wanted to get into a BME Masters program. Bio --> Engineering is not the most efficient path, plus the BS in Biology would lock you out of some graduate programs which do not accept applications from those with a BS in a discipline that’s not engineering or something closely related. If you’re seriously thinking of studying engineering, you’d be best served with an engineering BS.