When the solar summons you for the first time and introduces you to Sarevok she can also show you Jon.
I like this idea! But I don't think any sane PC would (or should) be able to have both Jonleth and Sarevok in the party. The two former enemies of the PC don't seem like they'd get along anyway. My thought here...
[Normal Sarevok speech, up until his revival]
[Jonleth Appears]
Jon: Ah, [CHARNAME]... I should've expected to see you here. I still have a small taint of your soul within me, I'm afraid.
[Solar never leaves until the first challenge is open]
Sarevok: You must be Irenicus! I've heard about you.
Jon: And I've heard about you. Sarevok, is it?
Sarevok: Bah! I don't need to be hearing a lesser being like yourself!
Solar: Both of your former enemies seem to want something. Let them speak.
[Insert PC agree or disagree with Solar here]
Sarevok: I would like a small piece of your Godly soul, [CHARNAME]! My mortal essence has been regained, but I need your help to get me out of here. If you would give me a drop of your soul, I think I can prove my worth to you!
Jon: And
I have a much better plan than that. The little bit of you I still have, [CHARNAME], brought me into this abyssal plane. I died by your hand, and I admit you are greater than I. I've been reduced to
this! But I will fight on your side! I can get you out of here if you learn to trust me!
Sarevok: Ha! I could help you out of here just as quickly!
Solar: This will be one of the first questions you'll have to answer. You cannot free both of them. Whom do you wish to fight by your side?
-Sarevok
Solar: Very well. You have chosen your fallen brother, Sarevok. Together, you must face your destiny.
[Solar opens up the first challenge and vanishes, taking Jonleth with her. Then Sarevok begins his "back to life" speech]
-Jon Irenicus
Solar: Very well. You have chosen the wizard who stole your very soul for his greedy purposes. With him at your side, I wish you luch, Bhaalchild.
[Solar opens up the first challenge and vanishes, taking Sarevok with her]
Jon: So we are to fight together this time. Perhaps I might reach my goal of redemption, with your help.
-I wish neither would join up with me! To the Nine Hells with them, once again!
Solar: Very well. Jonleth. Sarevok. I am to send you back into the Nine Hells.
[Solar opens up the first challenge and vanishes, taking Sarevok and Jonleth with her]
Every spelling error above is not my fault, and I should not be counted liable for such.