I haven't gotten a lifetime subscription yet but even if i do i will probably still pay for my monthly subscription because i want to see the game improve and that is how the devs get paid.
You cannot have two different subs on one account. You paid your monies with a LT, no need to buy a monthly too. The Devs will still get paid. If you feel that passionate about it though, perhaps you could by a monthly sub as a gift for a good friend or loved one.