It's possible to carry more then one (auto)rifle in a backpack. In some cases that sounds a bit complicated, if the rifles are to big and the the backpacks looking to small. If the rifles were disassembled and in this case not usable they could use the room of capacity in a better way. Otherwise the barrel of each rifle would peek out of the backpack.
If you can carry a NLAW on your back I'm sure that to carry a functional rifle in the same way is possible.

• review rifle capacity in bags
• use AT slot for secondary rifle
• disassembling of rifles