Is it reasonable to pronounce Zissel as “zees-le”/ “zee-zle” as to rhyme with [name_f]Liesel[/name_f]??
I love Zissel [name_f]Anouk[/name_f].
I’ve never heard of Zissel before! I have to say that I automatically read it as Ziss-el not Zees-el. However hopefully I’m in the minority and its just because I’m not familiar with the name.
I’m sure if you correct people, they will soon learn how to pronounce it.
I don’t think it’s too much of a problem at all (I’d assume it is supposed to be pronounced as you want it, just a variant spelling of Sisel/Zisel?), though I’d say be prepared to correct the majority, who would likely say ZISS-el.
I think that for the pronunciation you are after, one S would be better, having a few more people perhaps automatically using your preferred pronunciation (also, I do find the second S to be redundant and to me it looks much neater and nicer with a single S).
[name_f]Zisel[/name_f] [name_f]Anouk[/name_f] would be lovely. I actually really like this combo this way. 
As a pp said [name_f]Zisel[/name_f] would be more intuitive, most people will think “ziss” and not “zees/zeez” when they see the double s. If you prefer the double s it’s still totally usable, just might involve more correction.
I’d assume it was a spelling variation of Sissel which is pronounced more like “sizzle”, fast, not slow like [name_f]Liesl[/name_f]
I will echo what others said, that [name_f]Zisel[/name_f] might be more likely to get the pronunciation you’re looking for.
That being said, I’ve never actually met anyone with either name, so I’d go by whatever pronunciation was provided to me, and I wouldn’t be surprised by “zee-sle.”