Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is permissible for you to join Dhuhr and Asr prayers, either at the time of Dhuhr (praying Asr in advanced time) or at the time of Asr (praying Dhuhr at the last period of its fixed time), as long as you are a traveler.

The best is to do what is most suitable, either to pray Asr at an earlier time or delay Dhuhr until the last period of its fixed time.

There are many evidences about joining two prayers when traveling. Abu Dawud and others reported that "the Prophet (Sallallahu Alaihi wa Sallam) used to join Dhuhr and Asr, and Maghrib and Isha in the battle of Tabuk.

It is also a Sunnah to shorten the prayers as long as you are a traveler, i.e. the prayer of 4 Rak'ahs, should be offered in 2 Rak'ahs only. Allah says: "And when you (Muslims) travel in the land, there is no sin on you if you shorten your Salât (prayer)". (An-Nisa 4:101)

Moreover, Al-Bukhari and Muslim reported that Ibn Omar, (may Allah be pleased with him), said: "I accompanied the Prophet (Sallallahu Alaihi wa Sallam), and he did not offer, while traveling, more than 2 Rak'ahs, and so did Abu Bakr, Omar and Uthman, (may Allah be pleased with all of them.)

As regards the second question, we have already mentioned its answer above. The best when joining prayers is to do what is suitable. You can pray Asr at an advanced time with Dhuhr. Anas Ibn Malik , (may Allah be pleased with him), said: "The Prophet (Sallallahu Alaihi wa Sallam) when traveling  used to delay Dhuhr prayed until Asr time and prays them jointly if he started his journey before the sun was at its meridian. But if the sun was at its meridian before starting his journey, he prayed Dhuhr and started his journey." This is in case you are traveling. However, if you meant by your second question, that you want to join Dhuhr and Asr (at Dhuhr time) while you are still local, then a local person, in principle, should perform each prayer at its prescribed time. The fact that you cannot find a mosque to pray Asr in congregation is not an excuse to join Asr with Dhuhr if you are local.

All the earth is a mosque, as the Prophet (Sallallahu Alaihi wa Sallam) said: "The earth has been made for me and for my followers a place for offering Salat, and a thing to purify them (perform Tayammum)." (Reported by Al-Bukhari and Muslim, and narrated by Jabir.)

Allah knows best.