I bought a 120x120x120cm phototent for photographing clothing using mannequins. I already ordered 2x 5500K 150W(750W incandescent) daylight lamps, one for each side. I'm wondering if I will need more lighting due to the large size of the object. Shall I get another light placing it over or in front of the tent? If I light it from the front will it cast shadow on the backdrop? Also what camera settings shall I use? I have a Fujifilm Finepix S4300.
Hi Pavel and welcome to StackExchange, Photography.
A phototent is used to get even overall lighting of the subject.
The lighting, instead of hitting the subject directly, shines directly onto the translucent walls and ceiling of the phototent. The phototent in turn becomes the "source" providing indirect, diffuse and soft subject lighting.
Two of these light sources with a tent may be enough (amount of) light to make a shot; but, I don't think you'll be happy with the quality of the lighting.
A third source would be helpful to get even lighting of the phototent. You want even lighting. More wouldn't hurt; but, you are after flat lighting, more or less, with a light even background so a clipping path can be easily used to put the subject into layouts.
There should be no shadows cast on the walls of the inside of the tent so long as the sources remain outside the tent and their light does not fall onto the subject directly.
Take some care to prevent stray light shine into your camera lens during the session to reduce flare since you will be shooting toward the source.