Is the £7.5k in total or per tenant ?
I think the criteria is for the year, not the amount of tenants you have. So, for example if you have one person renting for 6 months, then another for following six months, potentially no tax to pay until £7500 is received.
If you have two people at the same time, I think other issues would crop up, such as licensing the property as an HMO.