Why does I-Photo and Photos open at the same time?

When does that happen? When you connect a camera or a Card Reader?

Or after restarting the Mac?

If it happens after the start-up, check the System Preferences > Users&Groups.

Select your account and open the Login Items. Deselect the applications you do not want to open at start-up.

If it happens when you connect a camera, open the iPhoto Preferences > General.

Set the preference "Connecting a camera opens" to "No Application".

Which of the two apps do you want to launch when you insert the card reader? iPhoto or Photos ?

léonie is referring to this menu in iPhoto's General preference pane:

To prevent Photos from launching:

When Photos opens after you connect your camera, card reader, or iPhone, disable the checkmark "Open Photos for this device". The checkmark will be below the toolbar on the left side of the Photos window. Then quit Photos, don't force quit, or it will not write the preferences file.

This checkmark will prevent Photos from opening until you format the card in the camera.

In iPhoto set "Connecting Camera Opens" to "iPhoto".