I'm not sure how to upload another picture under the same album for twitter, so I'll leave that to someone else
Go here and input the link to the tweet, then open the images you wish to upload in separate tabs and go through the usual process from there.
You can also make the process even easier by following the instructions here
Yukari-sama, what has captured your fervent interest this morning?Hold on a sec.It looks like she'll finally give me an opening ♪Just a bit, bit more...