Yes you can easily do that. For so many years we have been using 1 batter for both dosa and idlis. However, if you want to make your idlis more fluffier then you can use idly rava . To use idli rava, take 1 ratio of uri dal, 1.5 ratio of boiled rice and 1.5 ratio of idli rava. You need to squeeze of the excess water from the idli rava else the dough will be too water. Also, you can add little more Urad dal when using with rice to make more soft idlis. For example you can use 1.5 ratio of urad dal with 3 ratio of rice.