Also, when I had my Doppler at 16 weeks, mw couldn't tell if they were the same one or different (she sent me for an emergency scan - both fine!)... There was a slow one then a fast one, but in a similar position. Difference was about 30 bpm! She said they could be the same one as the heartbeat can increase like that after a movement. So one of your twins could have just had a little kick, hence the higher rate!
No I wouldn't have thought so, my 2 also had very different heartbeats (2 boys). If there were any concerns or issues she would have said something, maybe even referred you to the hospital. Congratulations and try not to worry.
20 week scan indicated that I am having 2 dd's. At my last 2 midwife appointments (22 and 25) she has managed to hear 2 heartbeats with doppler but one is significantly faster than the other (that's how she cld tell them apart). When I asked her if this was of concern, she brushed the question aside. Is this something to worry about?