I successfully used StickMount Pro (is needed to be able to connect more than one device) on my Android Phone and a small USB Hub connected with an OTG cable for a similar purpose. Obviously, two small card readers are also needed.
You have to try if your phone can power the Hub and the two readers, otherwise you need a powered hub. With a powered hub, it's even possible to attach external Hard drives (I was doing that).
Note: you need a rooted device.