It's be awful because Kakashi couldn't turn it off. Ever.
The strain two sharingan would put on kakashi would kill him. There's no way.
These. But i have a theory, that on his deathbed, obito will unlock rinnegan, in his sharingan eyes(so in kakashis socket, and in obitos socket, just like how he unlocked ms) So then kakashi can activate and deactivate rinnegan.(but kakashi could only use a few techniques, with his chakra level)
It is a misconception that you need ems to get rinnegan. The manga has only said you need senju dna and uchiha dna combined to get rinnegan. You still need to awaken it even if you have the requirements.
But it is likely my theory will turn out wrong, and kakashi will die, or stay the same power level.