Currency pairs

This is a simple idea based on asset allocation between two currencies. I choose USD and RUB because of great volatility during last and I think several next years. Mechanism is also simple: your portfolio consist of 50% RUB and 50% USD. Each 6 months you check currency rate USD/RUB, sell one currency and buy another to get the same 50/50 rate. You can start and finish to invest you money whenever you want, but for getting some profit period of investing can't be less then 2 years. Unfortunately, you cannot directly buy RUB. Yet, we can choose two ETFs, that depends on RUB and USD currencies rates. ETF is a good choice because it also allow us to diverse our money. Instead of USD we can initially choose IE0031442068 ISHARES S&P 500 and for RUB I could find DE000A1JXC86 HSBC MSCI Russia Capp.