I have been "stacking" batches and have attenuation issues. I will brew 5 G today and pitch the yeast. I repeat these steps tomorrow and add it to the top of the first batch, although I do not pitch more yeast. I also do not aerate either batch. I believe I need to aerate, but am unsure when to do it; 1st batch, 2nd batch, both batches. Also, should I pitch on the 2nd batch as well as the 1st? Any help would be great....Thanks
Aeration/oxygenation is key. Without it yeast cannot grow successfully, which is basically the whole idea of batch fermentation (read: brewing). So, as you suspect, you really do need to do it.
The best answer to this question depends on some missing information, specifically: what kind of yeast (liquid, dry, from a starter or previous batch) you're using; how much yeast you're pitching; and how you're aerating (oxygen injection, shaking, vigorous pouring). However, in general it should be fine to pitch and aerate just the first batch. You can aerate the second one too, depending on how much you oxygenate the first one, how long your lag phase is, what shape your yeast is in, and probably some other factors. If you're aerating by shaking, it probably couldn't hurt to do both. If you're directly injecting pure oxygen, I'd stick to just the first one, giving it a bit more than you would for a single batch. Don't wait until the second batch to aerate. If you're not seeing any/much fermentation activity by the time you add the second batch, it may be a good idea to aerate more.
If you were to pitch more on the second day, what you're essentially doing is introducing less healthy/ready-to-ferment yeast into a medium that already has yeast that's adapted and fermenting (hopefully) vigorously. Meaning, the newly pitched yeast is going to have trouble competing for nutrients.
I don't think there's any need to aerate the subsequent batches. Think about the purpose of aeration...the yeast use the oxygen to synthesize sterols which keep cell walls flexible for budding. But the more yeast you pitch, the less need there is for aeration since you have adequate amounts of yeast already. In your case, by the time you add the second batch you've had a lot of cell growth so further aeration shouldn't be necessary. You may have more of a wort issue than a yeast issue.