Well, it seems that you have got a couple of glasses . It could give you more choices for looks. However, what you should know is that if your myopia has been developed, then it would not be wise for you to wear your previous pair of glasses, because in this way, your eyes are not used to them anymore, and thus could have some dizzy feelings or sore eyes. Just take care of your eyes.
If the new eyeglasses and old eyeglasses are the same at the prescription, it will be ok for your eyes to change to wear whatever one. However, if your prescription of the new eyeglasses is higher than that of the old ones, you will feel the blurry at the vision when wearing the old eyeglasses. It is not suggested for you to wear the old ones which may affect your vision.
You would better not alternately wear your different eyeglasses. That is just because the new changed glasses need some days to adopt even the new one's prescription is equal to the old one, let alone the different prescription ones. Different prescription eyeglasses will damage your eyes' focus adjustment ability, which will make your prescription stronger than now in the future. If you feel hard to get used to the new glass, you can relax your eyes after work for 10 mins per hour. Looking around or distance, especially the green plants will be normal to use half month to get used to your new glasses.
VSP vision care can tell you whether wearing old glasses can weaken your eyes or not. You can have a look at http://www.youtube.com/watch?v=HUzF78ChxmA&list=PLC054D7C8C8A9583D I hope this can be useful to you. And there are many other topics about eye care, you can find what you like. good luck!
If your old glasses and your new glasses are of the same prescription and you just buy them for decoration. You can use your old glasses as well as new glasses. They will not cause any damages to your eyes. But if the new glasses have different degrees, it means that your eyesight decrease and new glasses can provide you with more clear vision than your old glasses at this time. You'd better throw away your old glasses because they may cause eye strain for you.
Since you have bought two pair of new eyeglasses, why not throw away the old eyeglasses? Old eyeglasses will not cause great damages to our eyes but they may lead to eye strain because they can't provide you with clear vision compared with new eyeglasses. So you'd better give up wearing your old eyeglasses.