As long as you use the same company's product (i.e., all Duncan Hines, or all Betty Crocker, etc.) the results will be okay. (I've actually combined as many as 3 mixes into a 1/2 sheetcake pan.)

If you want a marbled effect, start with the white mix first, and then swirl the lemon throughout. But if you want them combined together, you will need a very large bowl to do that in after you've made each mix, prior to pouring it into the cake pan(s). Otherwise you'll have some streaks in the final product.