Unless you really need it to be Jisho for some particular reason, can I just recommend a different resource?
https://www.kanjipedia.jp/ should have all the 旧字体 and 異体字 you want to search for.
Thanks！ I just tried the source you indicated and it doesn't seem to do what I'm looking for. For example, I input そう to get 双; the objective is to get the old form of 双. I'm not seeing any way to do that. All I get for そう is what appears to be the same list of options that I get on Jisho. And if I input the kanji 双, the site doesn't provide the old form.
(As to why I would prefer Jisho, I'm using it all the time and I prefer to keep things simple.）
Followup: this site
http://www2.japanriver.or.jp/search_kasen/search_help/refer_kanji.htm
does what I'm looking for. Just search (Ctrl-F) for either form (old or new) and the page will find and highlight it.
Hmm? The 旧字 appears right on the page for 双 though...
But sure, kanjipedia is not explicitly a resource for 旧字 like the other site you linked. It has them though.
How to find 旧字体・異体字 with jisho?
Jisho provides a few old/alternate forms of characters, but not many. Is there a way to find, say, the old form of 双 using jisho?