You don't have to disable membership auto renewal in other to be able to buy membership(s) manually. And as far as I know if you have a stacked membership at the end of a month then auto renew will not buy another one. But I'm not 100% sure about the latter point so you might want to confirm it with 48 support.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then please see here.