Is it possible to stack the effects of these items if you get multipule copies of a certain one?

Absolutely: http://wiki.prisonblock.com/Amenities

Just remember, the stacked effects are based on your base stats and not the modified stats. So if something adds 10% strength, and you have two of them, it will add 10% of your base strength and not 10% of your strength after adding the first 10%

Make sense?

the ones I was looking to stack where the ones that boosted NRG and STM since it boost your training.

Same rules apply though. Any percentage gains are calculated against your base stat when stacking