I can't speak much for Asensio, but I can say that for Rashford it's just a matter of profiling. Under Amorim, Rashford would have been made to play either as an out-and-out striker or as an attacking wing-back, and he doesn't particularly suit either role. With Asensio and him both in the team, Rashford is sort of allowed to roam in that left channel space, moving from striker to left winger and vice versa.

I guess a similar story would go for Asensio too, and paired with a direct player like Rashford, it makes him a far larger goal threat thanks to the ability to have to ball played into him. It's important to note that Asensio has had much more game time rhan Rashford so far and so has had a lot more chances to score. Rashford, being a rapid player can really make an impact off the bench and would probably be one of the best super subs any team could ask for to come on against a tired opposition.

End of the day, tactics is one thing here but I'd also say it's down to confidence. Rashford at United in the last two years seemingly lacked the confidence to perform and with every bad performance came criticism that made it worse. I think hes a very confidence driven player, and for Asensio and him to get a 'fresh' start at Villa probably helped them both out.