How can I have a concerted addition of bromines?
The proper term for concerted addition is syn addition.  I can't find any way of a direct way of producing a syn-addition product.  The only way I can figure you would have to do it is if you did the halogenation with water as the solvent you would get the halo-hydrin product with an anti-addition.  You could then follow this up with a bromination using $\ce{P,Br2}$ or $\ce{PBr3}$ which would invert the chirality and give you a syn-product.