Using を when not followed by a verb
But 「言{い}い」 and 「いいます」 are both verbs in:
「この面白{おもしろ}い話{はなし}を落語{らくご}と言い、落語{らくご}をする人{ひと}を落語家{らくごか}といいます。」
「言い」 is the continuative form and 「いいます」 is the masu-form of the verb 「言う」, isn't it?
Thus, 「言い」 is used mid-sentence because the sentence still continues after it.
「Noun + を」 does not need to be followed directly by a verb. There can be other words placed in between as in:
「ラーメンをおいしく食{た}べる」
「カーテンをゆっくり開{あ}ける」, etc.
Hope you are following this explanation.
「A + を + B + と + 言う」
BTW. is an extremely common set phrase meaning:
"to call A 'B'"
You ask:
Would it also work if it was simply:
「この面白い話「が」落語と言い、落語をする人「が」落語家といいます。」
No, it would not. That would not be natural-sounding under normal circumstances. Careful speakers would not use 「が」 in that sentence.