Afaik the campaign starter has to enable that (which not all of them do. Some of them are not even aware of that. Some replies were like. "You can do that only when you set up the campaign but it's too late for that now." Or. "I don't know how to do it." Some get the hang of it + change the neccessary criteria on the fly. When I placed my order on Friday morning just before going to work I couldn't choose multiple perks. EC payment should have been possible but my card number (though valid) wasn't accepted, so I used Visa instead.

I think it depends on the settings of the individual campaign as specified by the creator. So the experience may be different with just two/several projects on the very same platform. Heck, even a follow-up campaign by the same creator on the very same platform could use different settings if he/she gains more experience with it. That said I backed about 40 projects on Kickstarter, Indiegogo, Backerkit and other/individual sites.

Here I can imagine Chen not to be able to foresee why on earth one would want an extra sleeve if a cover is already included in the bundle. so why should he take that into consideration? So people manage to surprise creators anyway as they can have have a creative way of thinking sometimes. You simply cannot predict them. Life isn't like that... (use sleeve on Jolla tab I already own/order it as collector's item/use it with Aigo X86/ put Kindle Fire tablet into it - whatever...)