All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First we thank you for being keen to perform your prayers in congregation. We ask Allaah to help you and enable you to remain steadfast on this. As regards when it is only permissible for you to pray one prayer in congregation either Thuhr or 'Asr, then we think that you should pray Thuhr in congregation as it comes first, Allaah knows best. If you pray Thuhr individually so that you will be able to pray 'Asr in congregation, it might be that something may prevent you from doing so, and therefore you will miss praying Thuhr and 'Asr in congregation. We ask Allaah to take our souls while performing good deeds.
Besides, there is no doubt that there are verses and Prophetic narrations about the superiority of 'Asr prayer over Thuhr prayer, but in our view you should pray Thuhr first in congregation to be on the safe side as this is being quick in the race for performing good deeds.
Allaah knows best.