I would like this too. I think one reason it is unworkable is that the android runtime (and thus android apps) are designed to economize the battery usage. This is one of the reasons multitasking on android is clunky -- its not the main feature.

Purism made a phone that sort of does what you want. So they claim, but from what I know they are big and slow AF. I wouldn't know because I never got mine -- beware of that company. They will take your money and send you a defective piece of crap or nothing at all.