In true sense, both Yoga and Meditation are doorway to divine and can lead to self realization. Thousands of people have become enlightened following either yoga or meditation.
There is nothing lacking in any of the system. Yoga is a step by step guide to awaken your true potential. When we talk about 8 limbs of yoga then the seventh step is dhyana (meditation). So yoga prepares you to the state of meditation and then ultimately to samadhi.
On the other hand, if someone follows meditation sincerely then he will always pay attention to his body needs, he will do his daily tasks ethically and consciously. A genuine Meditator is content, honest, non-violent and practices celibacy. This relates to first two limbs of yoga. Moreover the mind comes automatically under control when you witness it. This relates to pratyahara (leading the mind inwards and thus preparing it for meditation) which is 5th limb of yoga.
So neither of the system lacks anything. Both are just different path to realize your inner nature. The question of using yoga and meditation together arises because of the current popularization of both the systems. e.g. yoga is popularized as a set of exercise to stay fit and healthy. Meditation is becoming popular to relax your mind or for good night sleep or to relive yourself from stress and tension.
This is like driving a racing car at 20 kmph. Both the system have immense potential and should be explored fully by sadhaks to get enlightened. One should go deep in any of the system but can any time take help of the other system also. Because in modern lifestyle we don’t use the body much and mind is getting too much used. So we need to use methods to bring balance and harmony in body and mind.
When you do yoga asanas then one should be conscious of the breathing and body movement. Do the body movements gracefully and consciously. Let your attention be totally on each body movement. Don’t do the yoga asanas with an absent mind. Be totally present there. So little introduction to meditation can easily enhance your yoga skills.
On the other hand, most of us live a sedentary lifestyle. So practicing sitting meditation technique in modern lifestyle is not easy. So if a meditator includes yoga asana as part of his life style then it won’t only help in having a healthy body but will also help in removing lethargy from the system. Taking few deep breaths before starting meditation process is very helpful in clearing the mind and preparing it for meditation session. Surya namaskar asana could be very helpful for meditator who do lot of sedentary work.
In fact we should not stick to only yoga and meditation, anything which helps you in your spiritual journey should be used and incorporated in the spiritual practice. e.g.
1. Vegetarianism is a good aid for spiritual journey.
2. One should try living a total life.
3. Celibacy is important in Spiritual path.
4. Practicing karma yoga in life
So make use of any spiritual practice which brings harmony in your body, mind and soul. Go deep either in yoga or in meditation and use the other system to have a healthy body and mind. Without healthy body you cannot excel in any of the system easily.