"... inflate me, too!", exclaimed Sara. "After all, why should you get to have all the fun?"

Joy managed to splutter, "B-but we don't have... any tanks left...". Apparently, being full of helium hinders one's ability to adequately breathe.
.
Sara contemplated something, than said, "Well, I'll just have to inflate using you, instead!"

If Joy had wanted to object to the decision, Sara's lightning-quick lip-lock silenced her easily. Sara began sucking air out of Joy via her mouth, but Joy, oddly, did not get any smaller, nor any less full or tight. She moaned as she felt her breasts grow larger, and felt her nipples hardening with arousal, and then inflating as well. She moaned louder with the immense pleasure of feeling her boobs fill up, and then slowly stretch and tighten from their bases down to their nipples. She felt her ass, hips, and thighs begin to expand when they ran out of space in her breasts, and felt her belly grow to the size equivalent of a pregnancy in its final stages and her innie belly button pop to an outie in the matter of a few seconds. Not long afterwards, she was as full as Joy was, though perhaps her breasts were just slightly larger.

(Now, this feels great, but we'll have to go down to normal size eventually...), thought Sara. "Hey, Joy, how about we...