I'm 27 wks. My midwife heard the heartbeat with a doppler monitor at bottom and top of bump. Could this be twins?

OB US will help. It can be easily diagnosed with the obstetrical ultrasound. I would encourage you to do so.
Possible. The doppler can pick up the single heartbeat of a singleton pregnancy from different locations on the abdomen depending on the angle that the doppler probe is held. Consider a table in the center of a dark room. You could find that table with a flashlight from different sides of the room by changing the direction of the light beam. If twins are suspected, a sonogram can confirm the # of babies.