It is preferable to lein the HafTarah from a Novi. Even a printed Novi is usable when it contains the entire sefer. The particular HafTarah printed in the Chumashim is not preferred. If the Shul does use a Novi for the HafTarah, each individual should read the HafTarah to himself along with the ba’al maftir. When the HafTarah is read from a Novi others should not read along out loud. Is there a source for this?
SA OC 284, MB quotes L'vush who wonders why a complete Navi is not used. Taz and Magen Avrohom hold that even printed on paper and not rolled is good. Magen Avrohom and Eiliya Rabba say that even so a complete Navi is required, and not what is included in the chumash.
MB continues: if a printed Navi is not available, there is to be lenient in order not to lose Haftorah reading. He then says that it is appropriate and proper for every shul to have a Navi written on Klaf (k'din).
Sha'ar Hatziyun 4 metions that it is expensive and so we cannot protest a shul that is lenient. However, in shuls where much money is spent on ornamentation, certainly they have a mitvah to purchase a complete Navi.
[From Dirshu note 2] Chazon Ish OC 60:11 writes there is no difference between a complete Navi (printed) and the Chumash.
As to those Klaf that have all the Haftoros on them, Aruch Hashulchan 66 writes that some places in Lithuania has the custom. Rabbi S"Z Auerbach in Shulchan Shlomo s'if katan 1, writes that if a congregation does not have a complete Navi on klaf, the Haftoros klaf is better than a chumash.
"If the Shul does use a Novi for the HafTarah, each individual should read the HafTarah to himself along with the ba’al maftir."
I do not know a source for this. I would even question it because of the rule that two voices are not heard. On a personal note, i find it very annoying.