There is a solution for this, as a lot of bikes don't have fixings for two bottle cages (I've never ridden one that has).
Hydration in sport is a serious subject, as even slight dehydration causes a significant reduction in power output. And it's not just running and cycling, as I first noticed it in fell walking where I 'lost interest' towards the end of a walk, and my then walking partner had a habit of falling over.
If your bike doesn't have two bottle cages, or you want to carry kit in the second one, I would recommend the extra large bottles. Below is a photo of two free, (if you spend more than 20 quid), bottles I have acquired over the last 10 years. I measured them last night, and the one on the left will hold 550ml if filled to the brim, whilst the one on the right will hold 800ml.
The good news is that the bigger bottle will fit a standard bottle cage, as pictured below
In very warm weather, or if you plan to ride over two hours, it is probably worth drinking 500ml before you start as well