Questions and answers

Why are there two currencies?

There are two foreign areas around you. One side is the area under the control of the USSR (Soviet) and second area is under the US control, at least financial in this case. So, as there are two foreign countries where you can buy and sell resources, you need to work with both currencies.

If you move the camera to an edge of the map you will see the border (Soviet or West) and the custom houses. Depending on where you purchase resources, you will spend Rubles (Soviet area) or Dollars (West area)

Why do I need money within a socialist society?

In a couple of words, you don't. In another, trade.

Rubles and dollars are only the foreign exchange for your republic. Your internal economy works without any currencies/money, only with resources.

Those two currencies are only present due to trade with foreign countries (Soviet or West). You can simply exchange resources - obtain resources which you need now, like at the start of the game. But you choose! You can focus solely on trade and getting money from natural resources that you will mine and sell to foreign countries... or build up a totally self-sufficient republic.