Hanging one picture is simple; add another picture and it may get confusing. The key to hanging pictures side by side, at an equal height and with the desired spacing in between, is precise measurements. Take your calculations and use a level to get pictures just right.
Select a height at which to hang the pictures. Base the height from the anchors on the back of the pictures to the ground. Leave a small pencil mark on the wall at that height.
Measure the width of each picture from its widest point using a tape measure. Figure out the distance apart of each picture from one another.
Calculate half of each picture's width plus the desired distance apart from one another to get the distance of how far apart each nail.
Place a level against the height marker on the wall. Make two more marks on the wall when the reading is level for the placement of nail.
Hammer a nail through a picture hanger at each mark. Secure each picture's anchor onto the hanger.
Confirm that the placement of both anchors on the back of the pictures are equal. If not, adjust calculations as necessary. Also, use identical nails and picture hangers for each picture.