Overlapping Visas

Ok,
we're moving forward with the battle for ancestry visa.

Our next obstacle is overlapping of visas.

My girlfriend, Sarah, has a student visa until the 31st Oct. She needs to get her ancestry visa before the end to enable to stay for the immediate time after that date. So her plan is to go to canada at the beginning of Oct and get her ancestry visa and then come in on that visa.

Is this allowed? Will she be allowed to have overlapping visas?

thanks in advance.

Michael and Sarah