Can I have two UK visas simultaneously?

I have a visitor UK visa valid until October 2017.

I also want to apply for a Tier 4 student visa. My studies in the UK start in September 2017.

If I apply for the student visa in June, will they simply issue an overlapping student visa, or will they cancel my visitor visa and I will not be able to come to the UK until September?

I am worried because in a similar situation with Schengen visas, my existing visa was canceled.