For Lara Croft and the Guardian of Light on the Xbox 360, a GameFAQs message board topic titled "Questions about Co-op".
Highlight the weapon you want and press Enter/A/X to place it in the selected slot. Repeat with the other two weapon slots. To switch weapons on the fly, see above. TOTEC'S SPEAR: If you play as Totec in the co-op game or as Lara in the single-player game, you'll be able to use the spear.
.