In case of 2 SIM cards on same mobile device please check: How do I install Viber.This is a list of identity card policies by country.
There may be a penalty for not carrying a card or other legally valid identification (a passport, for foreigners); in some cases a person may be detained until identity is proven. In some countries, police need a reason, such as crime suspicion or security risk.
In order to determine which is the fixed currency when neither currency is on the above list (i.e.
both are "other"), market convention is to use the fixed currency which gives an exchange rate greater than 1.000.
What you can do, however, is to register your second mobile device with a different number: your home number, an old cellphone number, family’s or friends…
The only requirement is that you are able to receive an activation SMS on it.
In other countries, police can ask for id without stating a reason. Normally there is an age limit, such as 18, from which the possession is mandatory, even if 15- to 17-year-olds might need a card in order to prove that they are under 18.