depends on your goal.
but I think it might be better to train a 18 SG now, and then a 18 center next year. at the end of the 2 years you'll have better SG and C then when you start now with both, since next year they both'll be 19.
but if it is your goal to increase the quality of your team steadily on both inside and outside fronts, I think it is perfectly doable...
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