They will co exist peacefully but theres a problem with the different requirement of water...the frontosa like hard water and the severums like acidic soft water... so for the extreme health of each fish i would try to follow their water requirements as close to thier needs as possible..
I agree with Liz. You CAN do it, especially if you have established CB LFS sevs who have been living/breeding in domestic hard/alkaline water for generations...but it's not ideal for either fish.