Yes there were at least two -main- muadhins but no they were not calling for prayer simultaneously as clearly stated in the second hadith.
The idea for the two adhans is that the first adhan for fajr is held before the time "Bilal pronounces the Adhan at night" so that people can wake up prepare themselves to go to the mosque etc. or as clearly described in the hadith to be prepared that a second call -on time- "till Ibn Um Maktum pronounces the Adhan" comes and that will be the call for the start of fast. This practice is still in use in many Muslim countries and considered as sunnah.
Beside Bilal ibn abi Rabah and 'Abdullah ibn abi Makthum (who was blind as stated in this hadith of ibn 'Omar and this of 'Aisha) there were other muadhins like abu Mahdurah al-Jamhi أبو محذورة الجمحي who has been ordered to call for the prayer in Mecca (see here) after the battle of Hunain, he later was the muadhin in the masjid al-Haraam he died in Mecca either 59 a.H. or 79 a.H. (according different reports).
Beside him there was Sa'ad ibn Quradh al-'Aaidh al-Ansari سعد القرظ بن عائذ who was appointed aas a muadhin in the mosque of quba' by the Prophet () and later as a muadhin for the masjid an-Nabawi and died at the time of al-hajaj ibn Yusuf a-Thaqafi (he was ordered by the Prophet () to call for prayer once Bilal was not in his company in this hadith from sunan ad-Daraqutni).
These four people are known to have been appointed to call for the prayer at the time of the Prophet (), but only Bilal and ibn Makthum did so in al-Masjid an-Nabawi.
Here a source for the four muadhins in Arabic