I want to buy Swagtastic Gear and a bandana and equip both of them but I'm scared that it will disable Swagtastic or the bandana and I would have only one equipped

you can have 1 3d item per body part, so if you won't be able to wear both at the same time as long as they're for the same body part

yeah you can only wear one item at each bodypart. Since swagtastic and bandana are both head items you can't wear them both at once.

edit: smag beat me to it ^^

When i tried to put my swag hat with the blindfold, it removed the blind fold. Same thing

deactivate both
put em both in a set
activate the set.

^this is a well known glitch that might allow you to wear both.

Last resort but you never know.