Well  you can make Dosa/Idli from Urad dal and rice powder much convinient too

For Dosa

-Take 3portion of Rice powder and 1portion of Urad dal powder and make better with worm water and let it ferment (duration will depend on temperature/weather)-

-Once fermented add salt and make dosa's

 

For Idli-Mix 2 portion of dal and 1.5 portionof rice and ferment ,once fermented add salt and make idlis

 

Hope this is helpful,do send feed back