It seems once Rabban Yochanan ben Zakkai cancelled the takanah and allowed the eidus the entire day the egg should be permitted in Chutz l'Aretz on the second day since the two day Rosh Hashanah will be like any other two day Yom Tov in Chutz l'Aretz - only one day is a true Yom Tov. So why did Rav and Shmuel assur the egg on the second day after the takanah was cancelled?
Avraham Sacks, Ramat Beit Shemesh
Rashi (DH Ha Lan) explains that in Chutz la'Aretz they kept two days of Rosh Hashanah because of the orignal Takanah, not because of the Safek. Since the Takanah of keeping two days had never ceased, they have to keep two days, which are considered as one long day.
Why didn't the takannah stop in chutz l'Aretz once Rabban Yochanan ben Zakkai cancelled the takanah?
Tosfos Eruvin 39a DH Ee Atem writes that the difference between Eretz Yisrael and Chutz La'aretz after the destruction of the Beis Hamikdash is that in Eretz Yisrael it was clear to everyone that the custom had changed, whilst in Chutz La'Aretz they did not possess a heker that anything had changed. This is because in Eretz Yisrael everyone knew that after the Churban they started accepting witnesses all day long so they understood why Rosh Hashanah was now kept as only one day. In contrast, in Chuta La'Aretz nothing had changed between before the destruction and after the destruction. Since there was no recognisable sign that anything had changed, it follows that the Halacha has to remain the same both before and after.