You don't need another lens yet because the two you've list are quite adequate. Probably more so the 70-300mm. Focusing on the eyes is paramount in portraiture. You can lock focus/meter and recompose.
You'll want to find a spot outside that is evenly shaded, no dappled shading as this creates uneven tones. Your best bet is an off camera flash to fill in the shadows of the eye sockets *read raccoon eyes*, but barring that, a reflector will certainly do the job. This may also require another person to assist positioning the reflector.
Have your subjects at least 6-10 feet away from any solid background. Stop down your lens in the f/4.5 to f/8 range so that entire depth of the subjects are within the focal plane. Unless of course you want to do a close up and have the focus start to blur behind the ears.
Your camera position should level to slightly higher than eye level. There are three basic mask positions..... 1) Full Face - ears are of equal size, 2) 2/3 mask - both eyes are seen, but nose does not break opposite cheek line and 3) Profile - exactly one half of the face is seen. Also remember that any position you want your subject to be in begins with the feet. If your subject is not balanced and comfortable it will show in your photos.
Regardless of how good your exposure and focus is, additional work in post processing is required to make the eyes pop. There are boo-coos of videos on youtube and other sources to search for your preferred method.
Some mask examples for you.....
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Hope some of this helps.