Off the top of my head, I do not know if Scott's topsoil is suitable - many of Scott's soil products (as well as other 'brand' name soil products) contain soil amendments e.g., peat moss, manure, etc. I can only recommend to read the label and avoid soil with additives.
In that you live in NJ, your sources for tigers are probably not 'breeders' - tigers are extremely difficult to breed in captivity. Thus your source(s) of animals is more than likely wild-caught - which is in itself a whole different discussion. But in that that is probably the case, there is always the risk that each would carry different organisms, depending on where collected, that could be pathogenic to one the other. The safest advice would be to quarantine each animal for a few months then introduce - though still has risk.
The 50L bin - just assure that the surface area is at least 360 sq in - equivalent of a 30 x 12 inch for 2 adult salamanders.